Basis of Presentation and Summary of Significant Accounting Policies (Details 4) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Carrying Amount
Summary of the fair value and carrying value of debt
Total debt	$ 1,642,847	$ 1,564,923
Carrying Amount | Fixed rate debt
Summary of the fair value and carrying value of debt
Total debt	1,026,847	814,423
Carrying Amount | Floating rate debt
Summary of the fair value and carrying value of debt
Total debt	616,000	750,500
Fair Value
Summary of the fair value and carrying value of debt
Total debt	1,675,000	1,618,000
Fair Value | Fixed rate debt
Summary of the fair value and carrying value of debt
Total debt	1,060,000	857,000
Fair Value | Floating rate debt
Summary of the fair value and carrying value of debt
Total debt	$ 615,000	$ 761,000