Notes Payable (Details) (USD $)	9 Months Ended			9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 2023 Notes [Member]	Apr. 30, 2013 2023 Notes [Member]	Sep. 30, 2013 Forward Contracts [Member] instrument	Apr. 30, 2013 Forward Contracts [Member] instrument
Debt Instrument [Line Items]
Original Principal					$ 350,000,000
Stated interest rate					3.30%
Proceeds from Issuance of Long-term Debt				347,406,000
Notes payable, unamortized discount	11,116,000		9,338,000		2,594,000
Effective Interest Rate					3.388%
Number of interest rate derivatives held						4	4
Derivative, Notional Amount						240,000,000	240,000,000
Interest rate cash flow hedge liability, fair value							3,156,000
Amount of cash flow hedge reported in AOCI							3,141,000
Proceeds from notes payable	347,406,000	320,011,000		344,266,000
Debt Issuance Cost				$ 3,140,000