Notes Payable (Details) (USD $)	6 Months Ended			6 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 2023 Notes	Apr. 30, 2013 2023 Notes	Jun. 30, 2014 2024 Notes	May 31, 2014 2024 Notes instrument	May 31, 2014 Forward Contracts instrument	Dec. 31, 2013 Forward Contracts instrument	Apr. 30, 2013 Forward Contracts instrument	Apr. 30, 2013 Forward Contracts 2023 Notes	May 31, 2014 Forward Contracts 2024 Notes
Debt Instrument [Line Items]
Original Principal					$ 350,000,000		$ 350,000,000
Stated interest rate					3.30%		3.90%
Proceeds from Issuance of Long-term Debt				347,406,000		349,293,000
Notes payable, unamortized discount	10,909,000		10,816,000		2,594,000		707,000
Effective Interest Rate					3.388%		3.924%
Number of interest rate derivatives held							3	3	4	4
Derivative, Notional Amount							225,000,000	225,000,000	240,000,000	240,000,000
Interest rate cash flow hedge liability, fair value										3,156,000	3,156,000
Amount of cash flow hedge reported in AOCI								6,312,000		3,141,000	3,141,000	6,312,000
Proceeds from notes payable	349,293,000	347,406,000		344,266,000		346,068,000
Debt Issuance Cost				$ 3,140,000		$ 3,225,000