Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended			3 Months Ended		6 Months Ended			3 Months Ended		6 Months Ended			0 Months Ended		0 Months Ended				6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Unsecured revolving credit facility	Jun. 30, 2013 Unsecured revolving credit facility	Jun. 30, 2014 Unsecured revolving credit facility	Jun. 30, 2013 Unsecured revolving credit facility	Aug. 10, 2014 Unsecured revolving credit facility	Jun. 30, 2014 Unsecured term loan	Jun. 30, 2013 Unsecured term loan	Jun. 30, 2014 Unsecured term loan	Jun. 30, 2013 Unsecured term loan	Jun. 30, 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%	Jun. 30, 2014 Senior Notes, due 2024 at 4.65%	Jun. 30, 2014 Convertible senior notes, due 2027 at 3.8%	Dec. 31, 2013 Convertible senior notes, due 2027 at 3.8%	Jun. 30, 2014 Revolving credit facility and term loan	Jun. 30, 2014 5.125% Senior Notes due 2015	Dec. 31, 2013 5.125% Senior Notes due 2015	Jul. 15, 2014 5.125% Senior Notes due 2015 Forecast
Indebtedness
Unsecured revolving credit facility				$ 750,000		$ 750,000
Unsecured term loan outstanding									400,000		400,000
Unsecured senior notes													2,355,000
Convertible senior notes	8,478		8,478															8,478
Current amount outstanding under revolving credit facility				40,000		40,000		40,000
Extendable term of credit facility						1 year
Amount available under revolving credit facility				710,000		710,000		710,000
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%	1.50%	1.25%	1.51%		1.35%	1.66%	1.36%	1.66%
Loss on extinguishment of debt	214																			214
Interest rate (as a percent)				1.26%		1.26%			1.35%		1.35%
Issuance of senior notes	2,345,527		2,295,527													350,000
Interest rate stated percentage														7.875%	7.875%	4.65%	4.65%	3.80%	3.80%		5.125%	5.125%	5.125%
Principal balance of debt repaid														300,000									280,000
Aggregate debt including accrued and unpaid interest redeemed														311,813
Proceeds from issuance of senior notes, net of underwriting discounts and other offering expenses	$ 346,616	$ 299,661														$ 345,949