Fair Value Measurements and Derivative Instruments - Credit Features (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015		Dec. 31, 2014
Derivative Instruments
Liability Derivatives	968,464		$ 767,635
Moody's, Baa3 Rating
Derivative Instruments
Credit ratings for senior debt	Baa3
Moody's, Ba1 Rating
Derivative Instruments
Credit ratings for senior debt	Ba1
Standard & Poor's, BBB- Rating | Minimum
Derivative Instruments
Credit ratings for senior debt	BBB-
Standard & Poor's, BB Rating
Derivative Instruments
Credit ratings for senior debt	BB
Interest rate contracts
Derivative Instruments
Number of derivative instruments	5
Designated as Hedging Instrument
Derivative Instruments
Liability Derivatives	933,584	[1]	767,635	[1]
Other long-term Liabilities | Designated as Hedging Instrument | Interest rate swaps
Derivative Instruments
Liability Derivatives	80,429	[1]	$ 65,768	[1]
Derivative Maturity More than Five Years | Minimum
Derivative Instruments
Maturity of interest rate instrument	5 years

[1] Accounting Standard Codification 815-20 “Derivatives and Hedging.”