Fair Value Measurements and Derivative Instruments - Credit Features (Details) (USD $) In Thousands, unless otherwise specified			3 Months Ended													3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Moody's, Baa3 Rating	Mar. 31, 2014 Moody's, Ba1 Rating	Mar. 31, 2014 Standard & Poor's, BBB- Rating Minimum	Mar. 31, 2014 Standard & Poor's, BB Rating	Mar. 31, 2014 Interest rate contracts hedge	Mar. 31, 2014 Designated as Hedging Instrument		Dec. 31, 2013 Designated as Hedging Instrument		Mar. 31, 2014 Other long-term Liabilities Designated as Hedging Instrument Interest rate swaps		Dec. 31, 2013 Other long-term Liabilities Designated as Hedging Instrument Interest rate swaps		Mar. 31, 2014 Derivative Maturity More than Five Years Minimum
Derivative instruments disclosure
Credit ratings for senior debt			Baa3	Ba1	BBB-	BB
Maturity of interest rate instrument																5 years
Liability Derivatives	$ 107,340	$ 100,260						$ 84,685	[1]	$ 77,629	[1]	$ 59,353	[1]	$ 66,920	[1]
Number of derivative instruments							5

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