Basis of Presentation and Summary of Significant Accounting Policies (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Carrying Amount
Financial Instruments
Total debt	$ 1,342,581	$ 1,300,295
Carrying Amount | Fixed rate debt
Financial Instruments
Total debt	690,581	690,295
Carrying Amount | Floating rate debt
Financial Instruments
Total debt	652,000	610,000
Fair Value
Financial Instruments
Total debt	1,303,000	1,222,000
Fair Value | Fixed rate debt
Financial Instruments
Total debt	650,000	611,000
Fair Value | Floating rate debt
Financial Instruments
Total debt	$ 653,000	$ 611,000