Annual report pursuant to Section 13 and 15(d)

Long-Term Incentive Plan (Details)

v3.8.0.1
Long-Term Incentive Plan (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2017
Apr. 17, 2017
Series B unit awards      
Assumptions:      
Units vested 32,875    
2017 LTIP      
Aggregate disclosure      
Expense recognized   $ 0.2  
Number of common shares may be delivered pursuant to awards, subject to customary adjustments     930,851
Awards granted   11,762  
Remaining shares granted 919,089 919,089  
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      
Expense recognized   $ 34.7  
Unamortized compensation expense related to nonvested Series B units 69.4 $ 69.4  
Weighted average period for recognizing unrecognized stock-based compensation expense   2 years  
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      
Expense recognized   $ 30.5  
Awards granted   92,000  
Awards forfeited   500  
IDR LLC | Series B Unit Awards classified as liability      
Assumptions:      
Floor equity value $ 3,900.0 $ 3,900.0  
Expected volatility (as a percent)   38.00%  
Risk free rate (as a percent)   2.36%  
Weighted average cost of capital (as a percent)   7.25%  
Estimated value (in dollars per unit)   $ 1,787  
Aggregate disclosure      
Expense recognized   $ 4.2  
Awards granted   8,000  
Awards forfeited   900