Long-term debt Schedule of Derivative Instruments (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Interest Rate Cap Agreements Effective June 29, 2018
Derivative [Line Items]
Derivative, effective date			Jun. 29, 2018
Derivative, expiration date			Jun. 30, 2020
Derivative Asset, Fair Value, Gross Asset	$ 28,000		$ 28,000		$ 851,000
Term Loan Facility | Interest Rate Cap One | Maximum
Derivative [Line Items]
Notional amounts of interest rate agreements	3,500,000,000		$ 3,500,000,000
LIBOR plus interest rate margin			3.50%
Cash Flow Hedging
Derivative [Line Items]
Amount of losses reclassified from accumulated OCI into income	(1,606,000)	$ (1,537,000)	$ (3,212,000)	$ (3,074,000)
Amount of unrecognized (losses) gains in OCI on interest rate cap agreements	31,000	268,000	611,000	(782,000)
Debt Expense | Cash Flow Hedging | Interest rate cap agreements
Derivative [Line Items]
Amount of losses reclassified from accumulated OCI into income	(2,163,000)	(2,070,000)	(4,326,000)	(4,140,000)
Amount of unrecognized (losses) gains in OCI on interest rate cap agreements	$ 42,000	$ 361,000	$ 823,000	$ (1,053,000)