Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Notes Payable [Line Items]
Notes payable	$ 583,944	$ 478,790
6.3% Senior Notes [Member]
Notes Payable [Line Items]
Notes payable	298,944	298,790
$600 Million Revolving Loan Facility [Member]
Notes Payable [Line Items]
Notes payable	$ 285,000	$ 180,000