Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended			3 Months Ended	9 Months Ended		3 Months Ended		7 Months Ended	9 Months Ended	0 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012	Sep. 30, 2013 Unsecured revolving credit facility	Sep. 30, 2013 Unsecured revolving credit facility	Sep. 30, 2012 Unsecured revolving credit facility	Sep. 30, 2013 Unsecured term loan	Sep. 30, 2012 Unsecured term loan	Sep. 30, 2012 Unsecured term loan	Sep. 30, 2013 Unsecured term loan	Jun. 06, 2013 Unsecured senior Notes, due 2023 at 4.5%
Indebtedness
Unsecured revolving credit facility				$ 750,000	$ 750,000
Extendable term of credit facility					1 year
Unsecured revolving credit facility, maximum borrowing capacity				1,500,000	1,500,000
Unsecured term loan, maximum borrowing capacity							500,000			500,000
Debt instrument, interest rate description					LIBOR					LIBOR
Margin over base rate, in basis points (as a percent)					1.30%					1.45%
Interest rate payable for facility fees (as a percent)					0.30%
Interest rate (as a percent)				1.48%	1.48%		1.64%			1.64%
Weighted average interest rate for borrowings (as a percent)				1.49%	1.50%	1.58%	1.64%	1.70%	1.70%	1.65%
Current amount outstanding under revolving credit facility				193,000	193,000	0
Amount available under revolving credit facility				557,000	557,000
Unsecured term loan outstanding							400,000			400,000
Issuance of senior notes	2,295,027		1,993,880								300,000
Interest rate stated percentage											4.50%
Proceeds from issuance of senior notes, net of underwriting discounts and other offering expenses	$ 299,661	$ 491,975									$ 297,115