Debt (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended		3 Months Ended		9 Months Ended		3 Months Ended	9 Months Ended		1 Months Ended	3 Months Ended	9 Months Ended			9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Convertible Senior Notes	Sep. 30, 2011 Convertible Senior Notes	Sep. 30, 2012 Convertible Senior Notes	Sep. 30, 2011 Convertible Senior Notes	Sep. 30, 2011 Convertible Notes Due 2011	Sep. 30, 2012 Convertible Notes Due 2011	Sep. 30, 2011 Convertible Notes Due 2011	Oct. 31, 2011 Convertible Notes Due 2016	Sep. 30, 2012 Convertible Notes Due 2016	Sep. 30, 2012 Convertible Notes Due 2016	Dec. 31, 2011 Convertible Notes Due 2016	Oct. 17, 2011 Convertible Notes Due 2016	Sep. 30, 2012 Mortgage Financing	Dec. 31, 2010 Mortgage Financing
Debt
Debt aggregate principal value												$ 250,000,000
Convertible Senior Notes, stated percentage													1.00%	1.00%	1.00%	1.00%
Amount by which aggregate conversion value exceeds par value													42,900,000
Conversion price (in dollars per share)												$ 47.69	$ 55.88	$ 55.88
Convertible debt, number of shares into which convertible												5.2
Percentage of closing sales price of the entity's common stock that conversion price must exceed in order for notes to be convertible														130.00%
Number of trading-days within 30 consecutive trading days in which closing price of the entity's common stock must exceed conversion price for notes to be redeemable														20 days
Trading-day period used to determine contingent conversion requirement on notes														30 days
Number of consecutive business days immediately after any five consecutive trading-day period during note measurement period														10 days
Number of consecutive trading-days during which the trading price of the convertible notes must be less than a given percentage of the entity's stock price														5 days
Percentage of trading price to product of last reported sale price of the entity's common stock and conversion rate, maximum														95.00%
Redemption price of convertible notes as a percentage of principal amount, due to change in control														100.00%
Equity component													57,900,000	57,900,000
Amortization schedule														5 years
Effective interest rate (as a percent)									7.50%	7.50%	7.50%			6.70%
Interest expense incurred in connection with our convertible notes
Contractual coupon rate of interest					625,000	312,000	1,875,000	937,000
Discount amortization					2,626,000	4,268,000	7,790,000	12,569,000
Interest expense-convertible notes					3,251,000	4,580,000	9,665,000	13,506,000
Carrying amount related to convertible notes
Principal balance													250,000,000	250,000,000	250,000,000
Convertible debt, discount to reduce net carrying value													(48,030,000)	(48,030,000)	(55,820,000)
Discount accumulated amortization													9,908,000	9,908,000	2,118,000
Carrying amount													201,970,000	201,970,000	194,180,000
Mortgage Financing
Debt financing under credit agreement																	70,000,000	70,000,000
Debt, maturity term																	48 months
Amortization schedule, period for calculation of principal payments																	25 years
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, alternative option (as a percent)																	2.75%
Debt, interest rate, alternative option, spread over federal funds effective rate (as a percent)																	0.05%
Debt, interest rate, alternative option, basis																	LIBOR
Debt, interest rate, alternative option, spread over basis (as a percent)																	1.00%
Interest expense
Interest expense	4,384,000	5,646,000	13,054,000	16,702,000
Less: interest capitalized		(230,000)	(905,000)	(446,000)
Total interest expense	$ 4,384,000	$ 5,416,000	$ 12,149,000	$ 16,256,000