Fair Value Measurements and Derivative Instruments - Credit Features (Details) $ in Thousands		6 Months Ended
		Jun. 30, 2015 USD ($) hedge	Dec. 31, 2014 USD ($)
Derivative Instruments
Liability Derivatives		$ 726,610	$ 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 | hedge		6
Designated as Hedging Instrument
Derivative Instruments
Liability Derivatives	[1]	$ 699,475	767,635
Other long-term Liabilities | Designated as Hedging Instrument | Interest rate swaps
Derivative Instruments
Liability Derivatives	[1]	$ 62,402	$ 65,768
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.”