Fair Value Measurements and Derivative Instruments - Income Statement Hedging Instruments (Details) (Fair Value Hedging, USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Effect of derivative instruments involved in hedging on the consolidated financial statements
Amount of Gain (Loss) Recognized in Income on Derivative	$ 18,128	$ 15,579
Amount of Gain (Loss) Recognized in Income on Hedged Item	(8,459)	(5,893)
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	2,976	3,069
Amount of Gain (Loss) Recognized in Income on Hedged Item	3,882	5,542
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	15,152	12,510
Amount of Gain (Loss) Recognized in Income on Hedged Item	$ (12,341)	$ (11,435)