Derivative Instruments and Hedging Activities (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Jun. 30, 2015	Dec. 31, 2014
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Derivative notional amount, cash flow hedge, received fixed pay variable interest rate swap	$ 710,000		$ 710,000
Derivative notional amount, cash flow hedge, forward starting interest rate swap	510,000		510,000
Derivative notional amount of cash flow hedge received, variable pay fixed interest rate swap	200,000		200,000		$ 62,500
Interest rate cash flow hedge gain (loss) to be reclassified during the next 12 months, net	5,200		5,200
Derivative transaction fees	$ 1,200	$ 874	$ 2,700	$ 1,300
Portion of fair value of outstanding interest rate swaps covered by collateral agreements (percent)	100.00%		100.00%			100.00%