Fair Value Measurements (Carrying Amount and Estimated Fair Values of Financial Instruments) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted cash		$ 8,542
Carrying Amount [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	25,685	53,583
Restricted cash		8,542
Unsecured revolving credit facility	230,000
Senior notes	1,668,934	1,669,473
Derivative instruments	264,201	287,878
Fair Value [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents	25,685	53,583
Restricted cash		8,542
Unsecured revolving credit facility	230,000
Senior notes	1,791,429	1,917,005
Derivative instruments	$ 264,201	$ 287,878