Derivatives - Interest Rate Risk (Narratives) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Aug. 31, 2017
Derivatives
Interest expense	$ 22,892	$ 21,365	$ 66,817	$ 62,842
Deferred pre-tax losses to be reclassified during next 12 months	$ 1,100		$ 1,100
Derivatives designated as hedging instruments: | Interest rate swaps
Derivatives
Weighted average effective fixed interest rate on interest rate swaps (as a percent)	1.80%		1.80%
Interest expense			$ 700	$ 0
Notional amount of interest rate swaps	$ 500,000		500,000
Derivatives designated as hedging instruments: | March 2022
Derivatives
Notional amount of interest rate swaps	$ 300,000		$ 300,000		$ 300,000
Derivative liability fair value					$ 700