Fair Value Measurements and Derivative Instruments - Income Statement Hedging Instruments (Details) - Fair Value Hedging - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Effect of derivative instruments involved in hedging on the consolidated financial statements
Amount of Gain (Loss) Recognized in Income on Derivative	$ (12,841)	$ 17,998	$ 5,287	$ 33,577
Amount of Gain (Loss) Recognized in Income on Hedged Item	18,273	(7,696)	9,814	(13,589)
Interest rate swaps | Interest expense, net of interest capitalized
Effect of derivative instruments involved in hedging on the consolidated financial statements
Amount of Gain (Loss) Recognized in Income on Derivative	2,872	3,067	5,848	6,136
Amount of Gain (Loss) Recognized in Income on Hedged Item	3,925	3,925	7,807	9,467
Interest rate swaps | Other income (expense)
Effect of derivative instruments involved in hedging on the consolidated financial statements
Amount of Gain (Loss) Recognized in Income on Derivative	(15,713)	14,931	(561)	27,441
Amount of Gain (Loss) Recognized in Income on Hedged Item	$ 14,348	$ (11,621)	$ 2,007	$ (23,056)