Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Notes Payable [Line Items]
Notes payable	$ 478,790	$ 298,637
6.3% Senior Notes [Member]
Notes Payable [Line Items]
Notes payable	298,790	298,637
$600 Million Revolving Loan Facility [Member]
Notes Payable [Line Items]
Notes payable	$ 180,000	$ 0