Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended			6 Months Ended	1 Months Ended	3 Months Ended		6 Months Ended		1 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 Unsecured revolving credit facility	Mar. 31, 2012 Unsecured term loan	Jun. 30, 2012 Unsecured term loan	Jun. 30, 2012 Unsecured term loan	Jun. 30, 2012 Unsecured term loan	Mar. 11, 2012 Unsecured term loan	Mar. 31, 2012 Convertible senior notes, due 2027 at 3.8%	Jun. 30, 2011 Convertible senior notes, due 2027 at 3.8%	Jun. 30, 2012 Convertible senior notes, due 2027 at 3.8%	Jun. 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%	Jun. 30, 2012 Senior Notes, due 2012 at 6.85%	Apr. 11, 2012 Senior Notes, due 2012 at 6.85%	Dec. 31, 2011 Senior Notes, due 2012 at 6.85%
Indebtedness
Unsecured revolving credit facility, available amount						$ 750,000
Unsecured revolving credit facility, maximum borrowing capacity						1,500,000
Unsecured revolving credit facility, maximum borrowing capacity								500,000	500,000	500,000
Extendable term of credit facility						1 year
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%
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%	6.85%	6.85%	6.85%
Repurchase of convertible senior notes												70,576
Estimated effective interest rate (as a percent)														6.06%
Net carrying amount	8,478		8,478		78,823									8,478			78,823
Unamortized discounts																	231
Non-cash amortization	1,376	1,508	2,954	3,009									398	270	790
Amount allocated as the equity component of the notes														37,710			37,710
Redemption of senior notes																			$ 100,829