Indebtedness (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended						1 Months Ended	10 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended				0 Months Ended			1 Months Ended			0 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Unsecured revolving credit facility	Dec. 31, 2011 Unsecured revolving credit facility	Dec. 31, 2010 Unsecured revolving credit facility	Mar. 31, 2012 Unsecured term loan	Dec. 31, 2012 Unsecured term loan	Dec. 31, 2012 Unsecured term loan	Mar. 12, 2012 Unsecured term loan	Mar. 31, 2012 Convertible senior notes, due 2027 at 3.8%	Dec. 31, 2012 Convertible senior notes, due 2027 at 3.8%	Dec. 31, 2011 Convertible senior notes, due 2027 at 3.8%	Dec. 31, 2010 Convertible senior notes, due 2027 at 3.8%	Mar. 20, 2012 Convertible senior notes, due 2027 at 3.8%	Apr. 11, 2012 Senior Notes, due 2012 at 6.85%	Dec. 31, 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%	Dec. 31, 2012 Senior Notes, due 2022 at 5%	Aug. 16, 2012 Senior Notes, due 2022 at 5%	Sep. 10, 2012 Senior Notes, due 2013 at 6.75%	Dec. 31, 2012 Senior Notes, due 2013 at 6.75%	Dec. 31, 2011 Senior Notes, due 2013 at 6.75%	Dec. 31, 2012 Senior Notes, due 2014 at 7.875%	Dec. 31, 2011 Senior Notes, due 2014 at 7.875%	Dec. 31, 2012 Senior Notes, due 2015 at 5.125%	Dec. 31, 2011 Senior Notes, due 2015 at 5.125%	Dec. 31, 2012 Senior Notes, due 2016 at 6.3%	Dec. 31, 2011 Senior Notes, due 2016 at 6.3%	Dec. 31, 2012 Senior Notes, due 2017 at 5.625%	Dec. 31, 2011 Senior Notes, due 2017 at 5.625%	Dec. 31, 2012 Senior Notes, due 2018 at 6.7%	Dec. 31, 2011 Senior Notes, due 2018 at 6.7%
Indebtedness
Unsecured revolving credit facility				$ 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
Debt instrument, interest rate description				LIBOR					LIBOR
Margin over base rate, in basis points (as a percent)				1.30%				1.45%	1.45%
Interest rate payable for facility fees (as a percent)				0.30%
Interest rate (as a percent)				1.51%				1.67%	1.67%
Weighted average interest rate for borrowings (as a percent)				1.56%	1.59%	0.90%		1.70%
Current amount outstanding under revolving credit facility				320,000
Amount available under revolving credit facility				430,000
Unsecured revolving credit facility	320,000	149,000
Term of loan							5 years
Unsecured term loan	400,000									400,000
Interest rate stated percentage												3.80%	3.80%		3.80%	6.85%	6.85%	6.85%		5.00%	5.00%	6.75%	6.75%	6.75%	7.875%	7.875%	5.125%	5.125%	6.30%	6.30%	5.625%	5.625%	6.70%	6.70%
Repurchase of convertible senior notes											70,576
Estimated effective interest rate (as a percent)												6.06%
Net carrying amount	8,478	78,823										8,478	78,823
Unamortized discounts	(11,120)	(5,169)										0	231
Non-cash amortization	6,179	6,305	7,123									270	1,605	2,602
Amount allocated as the equity component of the notes												37,710
Issuance of senior notes	1,993,880	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		50,000													100,829						287,000
Carrying amount	2,733,478											8,478	79,054					100,829		500,000				287,000	300,000	300,000	280,000	280,000	275,000	275,000	300,000	300,000	350,000	350,000
Initial exchange price (In dollars per share)												$ 50.50
Total unsecured senior notes	2,002,358	1,966,714
Total indebtedness	2,722,358	2,115,714
Required principal payments under outstanding debt
2014	300,000
2015	600,000
2016	275,000
2017	700,000
Thereafter	858,478
Total principal payments	$ 2,733,478											$ 8,478	$ 79,054					$ 100,829		$ 500,000				$ 287,000	$ 300,000	$ 300,000	$ 280,000	$ 280,000	$ 275,000	$ 275,000	$ 300,000	$ 300,000	$ 350,000	$ 350,000