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Debt (West Polaris facility - $420 million term loan and revolving credit facility agreements) (Details) - USD ($)
1 Months Ended 12 Months Ended
Aug. 31, 2018
Feb. 28, 2018
Aug. 31, 2017
Dec. 31, 2019
Jul. 31, 2020
Dec. 31, 2018
Jun. 30, 2015
Debt Instrument [Line Items]              
Debt outstanding       $ 2,878,200,000   $ 3,059,100,000  
West Polaris Facility | Revolving Credit Facility              
Debt Instrument [Line Items]              
Maximum borrowing capacity       420,000,000     $ 100,000,000
Amount outstanding             $ 226,000,000
Debt instrument, annual amortization payment       36,000,000      
Debt prepayment cost $ 9,400,000 $ 9,400,000 $ 37,400,000        
Debt outstanding       114,900,000      
Assets pledged as collateral       $ 502,500,000      
West Polaris Facility | Revolving Credit Facility | London Interbank Offered Rate (LIBOR)              
Debt Instrument [Line Items]              
Basis spread (as percent)       3.25%      
Commitment fee percentage       1.30%      
Scenario, Forecast | West Polaris Facility | Revolving Credit Facility              
Debt Instrument [Line Items]              
Balloon payment         $ 93,800,000