Annual report pursuant to Section 13 and 15(d)

Long-Term Incentive Plan (Details)

v3.10.0.1
Long-Term Incentive Plan (Details) - USD ($)
$ / shares in Units, $ in Millions
9 Months Ended 12 Months Ended
Dec. 31, 2018
Oct. 01, 2018
Sep. 30, 2018
Dec. 31, 2018
Dec. 31, 2017
Apr. 17, 2017
Aggregate disclosure            
Units vested 65,745     65,745 32,875  
Series B unit awards            
Assumptions:            
Units vested 65,745          
Aggregate disclosure            
Expense recognized       $ 34.4 $ 34.7  
Series B Unit Awards classified as equity | ASU 2018-07            
Long-Term Incentive Plan            
Effect of adoption   $ 3.0        
Series B Unit Awards classified as liability | ASU 2018-07            
Long-Term Incentive Plan            
Effect of adoption   $ (3.0)        
2017 AMGP LTIP            
Aggregate disclosure            
Expense recognized       $ 0.7 $ 0.2  
Number of common shares may be delivered pursuant to awards, subject to customary adjustments           930,851
Awards granted       49,225 11,762  
Remaining shares granted 881,626     881,626    
IDR LLC            
Long-Term Incentive Plan            
Percentage of amount of quarterly distribution in excess of threshold limit       6.00%    
Threshold limit for quarterly distribution       $ 7.5    
IDR LLC | Series B unit awards            
Long-Term Incentive Plan            
Authorized units 98,600     98,600    
Outstanding units 98,600     98,600    
Threshold limit for conversion of units to common shares $ 2,000.0     $ 2,000.0    
Assumptions:            
Vesting period       3 years    
Aggregate disclosure            
Unamortized compensation expense related to nonvested Series B units 34.4     $ 34.4    
IDR LLC | Series B unit awards | Maximum            
Long-Term Incentive Plan            
Percentage of amount of quarterly distribution in excess of threshold limit       6.00%    
Percentage of amount of equity value in excess of threshold limit for conversion of units to common shares       6.00%    
Percentage of newly issued common shares to the total number of issued and outstanding common shares allowed       6.00%    
IDR LLC | Series B Unit Awards classified as equity            
Assumptions:            
Floor equity value $ 2,000.0     $ 2,000.0    
Expected volatility (as a percent)       43.00%    
Risk free rate (as a percent)       2.45%    
Weighted average cost of capital (as a percent)       7.25%    
Estimated value (in dollars per unit)       $ 999    
Aggregate disclosure            
Awards forfeited       0 1,400  
IDR LLC | Series B Unit Awards classified as liability            
Assumptions:            
Floor equity value     $ 3,300.0      
Expected volatility (as a percent)     38.00%      
Risk free rate (as a percent)     3.00%      
Weighted average cost of capital (as a percent)     7.25%      
Estimated value (in dollars per unit)     $ 1,673