Debt (Details) (USD $) Share data in Millions, except Per Share data, unless otherwise specified	12 Months Ended								12 Months Ended				12 Months Ended					12 Months Ended	1 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2013 Convertible senior notes	Dec. 31, 2012 Convertible senior notes	Dec. 31, 2011 Convertible senior notes	Dec. 31, 2013 Revolving line of credit	Dec. 31, 2013 Revolving line of credit Maximum	Dec. 31, 2013 Revolving line of credit One-month LIBOR	Dec. 31, 2013 Convertible Notes Due 2016	Dec. 31, 2012 Convertible Notes Due 2016	Oct. 31, 2011 Convertible Notes Due 2016	Dec. 31, 2013 Convertible Notes Due 2011	Dec. 31, 2012 Convertible Notes Due 2011	Dec. 31, 2011 Convertible Notes Due 2011	Dec. 31, 2013 Mortgage Financing item	Dec. 31, 2010 Mortgage Financing	Dec. 31, 2013 Mortgage Financing One-month LIBOR	Nov. 30, 2011 Mortgage Financing -Midland Loan Services (PNC Bank N.A.)	Jun. 01, 2016 Mortgage Financing -Midland Loan Services (PNC Bank N.A.)
Debt
Maximum borrowing capacity							$ 75,000,000
Additional increase in borrowing capacity of the line of credit available at the entity's option								75,000,000
Debt, floating rate of interest basis									one month LIBOR									one month LIBOR
Debt, floating rate of interest, basis spread (as a percent)									0.50%									3.75%
Unused commitment fee (as a percent)							0.06%
Debt, maturity term							1 year									25 years			30 years
Debt aggregate principal value												250,000,000
Convertible Senior Notes, stated percentage										1.00%	1.00%	1.00%							6.35%
Conversion price (in dollars per share)										$ 113.08		$ 47.69
Convertible debt, number of shares into which convertible												5.2
Percentage of conversion price that the closing sales price of the entity's common stock must exceed in order to make the notes convertible										130.00%
Minimum 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 to be										5 days
Percentage of trading price of the entity's common stock's closing price and the conversion rate, maximum										95.00%
Amount by which aggregate conversion value exceeded par value										342,800,000
Redemption price of convertible notes as a percentage of principal amount, due to change in control										100.00%
Equity component										57,900,000
Amortization schedule										5 years			5 years	5 years	5 years
Effective interest rate (as a percent)										6.70%			7.50%	7.50%	7.50%
Interest expense incurred in connection with the entity's convertible notes
Contractual coupon rate of interest				2,500,000	2,500,000	1,510,000
Discount amortization				11,178,000	10,487,000	16,118,000
Interest expense-convertible notes				13,678,000	12,987,000	17,628,000
Carrying amount related to convertible notes
Principal balance				250,000,000	250,000,000											66,500,000
Convertible debt, discount to reduce net carrying value				(34,155,000)	(45,333,000)
Discount, accumulated amortization				23,783,000	12,605,000
Carrying amount				215,845,000	204,667,000											70,337,000
Mortgage Financing
Mortgage loan under credit agreement																	70,000,000		3,600,000
Mortgage term							1 year									25 years			30 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									one month LIBOR
Debt, floating rate of interest, basis spread (as a percent)									0.50%									3.75%
Debt, interest rate at period end (as a percent)																3.90%
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%
Maximum consolidated leverage ratio																2.5
Financial covenants, measurement period (in rolling quarters)																4
Financial covenants, minimum liquidity amount																150,000,000
Financial covenants, portion of vested STAP awards not included in calculation																50,000,000
Balance of debt remaining at maturity date																				3,400,000
Future maturities relating to mortgage financings
2014																66,636,000
2015																97,000
2016																3,490,000
2017																114,000
Carrying amount				215,845,000	204,667,000											70,337,000
Interest expense
Interest expense	18,117,000	17,544,000	22,209,000
Less: interest capitalized	(59,000)	(905,000)	(842,000)
Total interest expense	$ 18,058,000	$ 16,639,000	$ 21,367,000