Long-term debt - Effects of Interest Rate Swap and Cap Agreements (Detail) - Cash Flow Hedging - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Derivative Instruments Gain Loss [Line Items]
Amount of unrecognized gains (losses) in OCI on interest rate cap agreements	$ 37	$ (478)	$ 819	$ (5,479)
Amount of losses reclassified from accumulated OCI into income	1,606	1,265	4,680	3,793
Tax benefit (expense)
Derivative Instruments Gain Loss [Line Items]
Amount of unrecognized gains (losses) in OCI on interest rate cap agreements	(13)	304	(284)	3,488
Amount of losses reclassified from accumulated OCI into income	(557)	(805)	(1,623)	(2,415)
Interest rate cap agreements | Debt Expense (Including Refinancing Charges)
Derivative Instruments Gain Loss [Line Items]
Amount of unrecognized gains (losses) in OCI on interest rate cap agreements	50	(782)	1,103	(8,967)
Amount of losses reclassified from accumulated OCI into income	$ 2,163	$ 2,070	$ 6,303	$ 6,208