Fair Value Measurements and Derivative Instruments - Income Statement Hedging Instruments (Details) - Fair Value Hedging - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Effect of derivative instruments involved in hedging on the consolidated financial statements
Amount of Gain (Loss) Recognized in Income on Derivative	$ 25,134	$ 53	$ 30,420	$ 33,630
Amount of Gain (Loss) Recognized in Income on Hedged Item	(16,284)	7,902	(6,470)	(5,687)
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	2,793	3,063	8,640	9,199
Amount of Gain (Loss) Recognized in Income on Hedged Item	3,968	3,968	11,775	13,435
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	22,341	(3,010)	21,780	24,431
Amount of Gain (Loss) Recognized in Income on Hedged Item	$ (20,252)	$ 3,934	$ (18,245)	$ (19,122)