Fair Value Measurements and Derivative Instruments - Income Statement Hedging Instruments (Details) (Fair Value Hedging, USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Effect of derivative instruments involved in hedging on the consolidated financial statements
Amount of Gain (Loss) Recognized in Income on Derivative	$ 53	$ 3,980	$ 33,630	$ (51,987)
Amount of Gain (Loss) Recognized in Income on Hedged Item	7,902	8,883	(5,687)	83,655
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	3,063	3,022	9,199	6,299
Amount of Gain (Loss) Recognized in Income on Hedged Item	3,968	9,503	13,435	28,102
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	(3,010)	958	24,431	(58,286)
Amount of Gain (Loss) Recognized in Income on Hedged Item	$ 3,934	$ (620)	$ (19,122)	$ 55,553