Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Indebtedness
Net carrying amount	$ 8,478		$ 8,478
Non-cash amortization	1,512	1,578
Unsecured revolving credit facility
Indebtedness
Unsecured revolving credit facility	750,000
Extendable term of credit facility	1 year
Unsecured revolving credit facility, maximum borrowing capacity	1,500,000
Debt instrument, interest rate description	LIBOR
Margin over base rate, in basis points (as a percent)	1.30%
Interest rate payable for facility fees (as a percent)	0.30%
Interest rate (as a percent)	1.51%
Weighted average interest rate for borrowings (as a percent)	1.51%	1.59%
Current amount outstanding under revolving credit facility	10,000
Amount available under revolving credit facility	740,000
Unsecured term loan
Indebtedness
Unsecured term loan, maximum borrowing capacity	500,000
Debt instrument, interest rate description	LIBOR
Margin over base rate, in basis points (as a percent)	1.45%
Interest rate (as a percent)	1.66%
Weighted average interest rate for borrowings (as a percent)	1.66%	1.70%
Unsecured term loan outstanding	$ 400,000