Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		3 Months Ended			3 Months Ended			0 Months Ended		0 Months Ended				3 Months Ended
	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2014 Unsecured revolving credit facility	Mar. 31, 2013 Unsecured revolving credit facility	May 05, 2014 Unsecured revolving credit facility Subsequent event	Mar. 31, 2014 Unsecured term loan	Mar. 31, 2013 Unsecured term loan	Mar. 31, 2014 Unsecured senior notes	Feb. 15, 2014 Senior Notes, due 2014 at 7.875%	Dec. 31, 2013 Senior Notes, due 2014 at 7.875%	Mar. 12, 2014 Senior Notes, due 2024 at 4.65%	Mar. 31, 2014 Senior Notes, due 2024 at 4.65%	Mar. 31, 2014 Convertible senior notes, due 2027 at 3.8%	Dec. 31, 2013 Convertible senior notes, due 2027 at 3.8%	Mar. 31, 2014 Revolving credit facility and term loan
Indebtedness
Unsecured term loan outstanding						$ 400,000
Unsecured senior notes								2,355,000
Convertible senior notes	8,478	8,478											8,478
Current amount outstanding under revolving credit facility			0		0
Extendable term of credit facility			1 year
Amount available under revolving credit facility			750,000		750,000
Maximum borrowing capacity that may be increased															2,300,000
Debt instrument, interest rate description			LIBOR			LIBOR
Margin over base rate before amendment, in basis points (as a percent)			1.30%			1.45%
Margin over base rate, in basis points (as a percent)			1.10%			1.20%
Interest rate payable for facility fees before amendment, (as a percent)			0.30%
Interest rate payable for facility fees (as a percent)			0.20%
Weighted average interest rate for borrowings (as a percent)			1.25%	1.51%		1.38%	1.66%
Loss on extinguishment of debt	(214)														(214)
Interest rate (as a percent)						1.35%
Issuance of senior notes	2,345,151	2,295,527									350,000
Interest rate stated percentage									7.875%	7.875%	4.65%	4.65%	3.80%	3.80%
Principal balance of debt repaid									300,000
Debt redeemed									311,813
Proceeds from issuance of senior notes, net of underwriting discounts and other offering expenses	$ 346,616										$ 345,999