Notes Payable (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Notes Payable [Line Items]
Notes payable	$ 654,059	$ 583,944
6.3% Senior Notes [Member]
Notes Payable [Line Items]
Notes payable	299,059	298,944
$600 Million Revolving Loan Facility [Member]
Notes Payable [Line Items]
Notes payable	$ 355,000	$ 285,000