Financial Instruments Measured at Fair Value - Summary of Effects on Consolidated Financial Statements of Designated As Cash Flow Hedges (Details) - Cash Flow Hedges - Designated - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2018	Dec. 31, 2016
Interest Rate Swaps
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss) (effective portion)	$ (3,388)	$ 9,838	$ (8,331)
Interest Rate Swaps | Interest Expense
Derivative Instruments Gain Loss [Line Items]
Loss recognized in Interest expense or cost of services provided (ineffective portion)	(6)
Net gain (loss) reclassified from Accumulated other comprehensive loss into Interest expense or cost of services provided	(7,424)	(9,816)	(4,947)
Fuel Swap Contracts
Derivative Instruments Gain Loss [Line Items]
Income (loss) recognized in Other comprehensive income (loss) (effective portion)	(641)	57	666
Fuel Swap Contracts | Cost of Services Provided
Derivative Instruments Gain Loss [Line Items]
Loss recognized in Interest expense or cost of services provided (ineffective portion)	19		301
Net gain (loss) reclassified from Accumulated other comprehensive loss into Interest expense or cost of services provided	$ (398)	$ 7	$ (3,767)