Debt (Details) (USD $)	3 Months Ended		9 Months Ended		1 Months Ended		3 Months Ended		9 Months Ended			9 Months Ended	12 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010	Oct. 31, 2011 2011 Convertible Senior Notes	Oct. 30, 2006 2011 Convertible Senior Notes Y	Sep. 30, 2011 2011 Convertible Senior Notes	Sep. 30, 2010 2011 Convertible Senior Notes	Sep. 30, 2011 2011 Convertible Senior Notes	Sep. 30, 2010 2011 Convertible Senior Notes	Dec. 31, 2010 2011 Convertible Senior Notes	Sep. 30, 2011 Mortgage Financing Y	Dec. 31, 2010 Mortgage Financing M
Debt
Debt issued at par value						$ 250,000,000
Payment of outstanding principal amount to note holders					250,000,000
Payment of accrued interest to note holders					625,000
Period for determining the number of shares to be issued upon maturity (in days)					20
Closing price of entity's common stock at period end (in dollars per share)							$ 37.49		$ 37.49
Equity component, estimated fair value						72,400,000
Period over which discount is being amortized under interest method						5
Effective interest rate (as a percent)									7.50%
Interest expense related to 2011 convertible senior notes
Contractual coupon rate of interest							312,000	312,000	937,000	937,000
Discount amortization							4,268,000	3,962,000	12,569,000	11,670,000
Effective interest-Convertible Senior Notes							4,580,000	4,274,000	13,506,000	12,607,000
Carrying amount related to 2011 convertible senior notes
Principal balance							249,968,000		249,968,000		249,968,000
Convertible debt, discount to reduce net carrying value							(1,431,000)		(1,431,000)		(14,000,000)
Discount accumulated amortization							70,971,000		70,971,000		58,402,000
Carrying amount							248,537,000		248,537,000		235,968,000
Mortgage Financing
Debt financing under credit agreement													70,000,000
Debt, maturity term (in months)													48
Amortization schedule, period for calculation of principal payments (in years)												25
Amortization schedule, fixed interest rate for calculation of principal payments (as a percent)												7.00%
Debt, floating rate of interest basis												one month LIBOR
Debt, floating rate of interest, basis spread (as a percent)												3.75%
Debt, interest rate at period end (as a percent)												4.00%
Debt, interest rate terms, alternative option												Option to change the rate of interest charged on the loan to 2.75 percent plus the greater of: (1) Wells Fargo's prime rate, or (2) the federal funds effective rate plus 0.05 percent, or (3) LIBOR plus 1.0 percent.
Debt, prepayment premium within first six-months of the term, percentage of loan balance (as a percent)												1.50%
Debt, prepayment premium, successive six-month intervals, decline in percentage of loan balance (as a percent)												0.50%
Interest expense
Interest expense	5,646,000	4,836,000	16,702,000	14,282,000
Less: interest capitalized	(230,000)	(27,000)	(446,000)	(27,000)
Total interest expense	$ 5,416,000	$ 4,809,000	$ 16,256,000	$ 14,255,000