Long-Term Debt - Effects of Interest Rate Swap and Cap Agreements (Details) - Cash Flow Hedging - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of unrealized losses in OCI on interest rate cap and swap agreements	$ (5,437)	$ (3,670)	$ (12,241)
Amount of losses reclassified from accumulated OCI into income	5,058	2,566	3,111
Tax benefit (expense)
Derivative Instruments, Gain (Loss) [Line Items]
Amount of unrealized losses in OCI on interest rate cap and swap agreements	3,460	2,343	7,844
Amount of losses reclassified from accumulated OCI into income	(3,220)	(1,632)	(1,992)
Interest rate cap agreements | Debt Expense (Including Refinancing Charges)
Derivative Instruments, Gain (Loss) [Line Items]
Amount of unrealized losses in OCI on interest rate cap and swap agreements	(8,897)	(5,198)	(16,114)
Amount of losses reclassified from accumulated OCI into income	8,278	3,899	2,439
Interest rate swap agreements | Debt Expense (Including Refinancing Charges)
Derivative Instruments, Gain (Loss) [Line Items]
Amount of unrealized losses in OCI on interest rate cap and swap agreements	0	(815)	(3,971)
Amount of losses reclassified from accumulated OCI into income	$ 0	$ 299	$ 2,664