Indebtedness (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended				3 Months Ended		9 Months Ended			1 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011 Y agreement property	Sep. 30, 2010	Nov. 30, 2011	Oct. 24, 2011	Sep. 30, 2011 Convertible senior notes, due 2027 at 3.8%	Sep. 30, 2010 Convertible senior notes, due 2027 at 3.8%	Sep. 30, 2011 Convertible senior notes, due 2027 at 3.8%	Sep. 30, 2010 Convertible senior notes, due 2027 at 3.8%	Dec. 31, 2010 Convertible senior notes, due 2027 at 3.8%	Jan. 31, 2011 Mortgage note, due 2011 at 8.3%	Sep. 30, 2011 Mortgage note, due 2011 at 8.3%	Jan. 03, 2011 Mortgage note, due 2011 at 8.3%	Dec. 31, 2010 Mortgage note, due 2011 at 8.3%
Indebtedness
Interest rate stated percentage (as a percent)	1.53%		1.53%				3.80%		3.80%		3.80%		8.30%	8.30%	8.30%
Estimated effective interest rate (as a percent)							6.06%		6.06%
Net carrying amount							$ 78,480		$ 78,480		$ 77,484
Unamortized discount							574		574		1,570
Amortization of deferred financing costs and debt discounts as interest	1,614	1,488	4,623	5,629			404	380	1,194	2,217
The amount allocated as the equity component of the notes							37,710		37,710
Unsecured revolving credit facility, maximum borrowing capacity	750,000		750,000		750,000	750,000
Extendable term of credit facility (in years)			1
Unsecured revolving credit facility, interest rate basis			LIBOR plus a spread
Unsecured revolving credit facility, interest rate payable (as a percent)	1.30%		1.30%
Unsecured revolving credit facility, amount outstanding	115,000		115,000
Unsecured revolving credit facility, available amount	635,000		635,000
Principal balance of debt repaid												$ 3,383