Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended			9 Months Ended	1 Months Ended	3 Months Ended	7 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended		9 Months Ended				0 Months Ended			1 Months Ended			1 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011	Sep. 30, 2012 Unsecured revolving credit facility	Mar. 31, 2012 Unsecured term loan	Sep. 30, 2012 Unsecured term loan	Sep. 30, 2012 Unsecured term loan	Sep. 30, 2012 Unsecured term loan	Mar. 12, 2012 Unsecured term loan	Mar. 31, 2012 Convertible senior notes, due 2027 at 3.8%	Sep. 30, 2012 Convertible senior notes, due 2027 at 3.8%	Sep. 30, 2011 Convertible senior notes, due 2027 at 3.8%	Sep. 30, 2012 Convertible senior notes, due 2027 at 3.8%	Sep. 30, 2011 Convertible senior notes, due 2027 at 3.8%	Mar. 20, 2012 Convertible senior notes, due 2027 at 3.8%	Dec. 31, 2011 Convertible senior notes, due 2027 at 3.8%	Apr. 11, 2012 Senior Notes, due 2012 at 6.85%	Sep. 30, 2012 Senior Notes, due 2012 at 6.85%	Dec. 31, 2011 Senior Notes, due 2012 at 6.85%	Aug. 31, 2012 Senior Notes, due 2022 at 5%	Sep. 30, 2012 Senior Notes, due 2022 at 5%	Aug. 16, 2012 Senior Notes, due 2022 at 5%	Sep. 30, 2012 Senior Notes, due 2013 at 6.75%	Sep. 10, 2012 Senior Notes, due 2013 at 6.75%	Dec. 31, 2011 Senior Notes, due 2013 at 6.75%
Indebtedness
Unsecured revolving credit facility, available amount						$ 750,000
Extendable term of credit facility						1 year
Unsecured revolving credit facility, maximum borrowing capacity						1,500,000
Unsecured term loan, maximum borrowing capacity								500,000	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%	1.45%	1.45%
Interest rate payable for facility fees						0.30%
Weighted average interest rate for borrowings (as a percent)						1.58%		1.70%	1.70%
Term of loan							5 years
Unsecured term loan											400,000
Interest rate stated percentage													3.80%	3.80%	3.80%	3.80%	3.80%	3.80%	6.85%	6.85%	6.85%		5.00%	5.00%	6.75%	6.75%	6.75%
Repurchase of convertible senior notes												70,576
Estimated effective interest rate (as a percent)													6.06%		6.06%
Net carrying amount	8,478		8,478		78,823								8,478		8,478			78,823
Unamortized discounts													0		0			231
Non-cash amortization	1,694	1,614	4,648	4,623									0	404	270	1,194
Amount allocated as the equity component of the notes													37,710		37,710			37,710
Issuance of senior notes	1,993,387		1,993,387		1,887,891																			500,000
Proceeds from issuance of senior notes, net of underwriting and other offering expenses			491,975																			487,946
Repayment of senior notes			$ 387,829																$ 100,829						$ 287,000