Fair Value Measurements and Disclosures Concentration of Risk (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015
Concentration Risk [Line Items]
Fair Value of Derivative Assets	$ 316,394
Canadian Imperial Bank of Commerce [Member]
Concentration Risk [Line Items]
Fair Value of Derivative Assets	68,625	[1]
JPMorgan Chase Bank [Member]
Concentration Risk [Line Items]
Fair Value of Derivative Assets	93,642	[1]
Wells Fargo Bank [Member]
Concentration Risk [Line Items]
Fair Value of Derivative Assets	46,899	[1]
Natixis [Member]
Concentration Risk [Line Items]
Fair Value of Derivative Assets	43,640	[1]
Bank of Nova Scotia [Member]
Concentration Risk [Line Items]
Fair Value of Derivative Assets	30,479	[1]
Key Bank [Member]
Concentration Risk [Line Items]
Fair Value of Derivative Assets	24,314	[1]
Other Lenders in Our Credit Facility [Member]
Concentration Risk [Line Items]
Fair Value of Derivative Assets	$ 8,795

[1] Major lender in our revolving credit facility. See Note 7, Long-Term Debt.