Debt (Details) (USD $)	12 Months Ended			3 Months Ended	1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2014	Feb. 11, 2015	Dec. 31, 2014	Oct. 31, 2011	Dec. 31, 2010
Interest expense
Interest expense	$ 17,592,000	$ 18,117,000	$ 17,544,000
Less: interest capitalized		(59,000)	(905,000)
Total interest expense	17,592,000	18,058,000	16,639,000
Revolving line of credit
Debt
Maximum borrowing capacity	75,000,000			75,000,000		75,000,000
Unused commitment fee (as a percent)	0.06%
Outstanding amount	0			0		0
Revolving line of credit | Maximum
Debt
Additional increase in borrowing capacity of the line of credit available at the entity's option	75,000,000			75,000,000		75,000,000
Revolving line of credit | One-month LIBOR
Debt
Debt, floating rate of interest basis	one-month LIBOR
Debt, floating rate of interest, basis spread (as a percent)	0.50%
Mortgage Financing
Debt, floating rate of interest basis	one-month LIBOR
Debt, floating rate of interest, basis spread (as a percent)	0.50%
Convertible Notes Due 2016
Debt
Debt aggregate principal value							250,000,000
Convertible Senior Notes, stated percentage	1.00%	1.00%	1.00%	1.00%		1.00%	1.00%
Conversion price (in dollars per share)	$ 129.49			$ 129.49		$ 129.49	$ 47.69
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%
Percentage of conversion price that the closing sales price of the entity's common stock exceeded	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	238,000,000
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		57,900,000
Amortization schedule	5 years
Effective interest rate (as a percent)	6.70%			6.70%		6.70%
Interest expense incurred in connection with the entity's convertible notes
Contractual coupon rate of interest	2,151,000	2,500,000	2,500,000
Discount amortization	11,057,000	11,178,000	10,487,000
Interest expense-convertible notes	13,208,000	13,678,000	12,987,000
Carrying amount related to convertible notes
Principal balance	138,750,000	250,000,000		138,750,000		138,750,000
Convertible debt, discount to reduce net carrying value	(12,336,000)	(34,155,000)		(12,336,000)		(12,336,000)
Discount, accumulated amortization	19,819,000	23,783,000		19,819,000		19,819,000
Carrying amount	126,414,000	215,845,000		126,414,000		126,414,000
Shares issued for conversion of 2016 convertible notes (in shares)				1,500,000
Value of convertible note settled due to conversion requests				111,300,000
Extinguishment loss				(4,600,000)
Number of common shares bought under note hedge				1,500,000
Number of shares for future conversion	2,900,000
Mortgage Financing
Effective interest rate (as a percent)	6.70%			6.70%		6.70%
Future maturities relating to mortgage financings
Carrying amount	126,414,000	215,845,000		126,414,000		126,414,000
Convertible Notes Due 2016 | Subsequent Event [Member]
Carrying amount related to convertible notes
Shares issued for conversion of 2016 convertible notes (in shares)					193,000
Value of convertible note settled due to conversion requests					14,000,000
Extinguishment loss					513,000
Number of common shares bought under note hedge					193,000
Number of shares for future conversion					2,600,000
Convertible Notes Payable					124,800,000
Mortgage Financing - Wells Fargo Bank
Mortgage Financing
Mortgage loan under credit agreement								70,000,000
Amortization schedule, fixed interest rate for calculation of principal payments (as a percent)	7.00%			7.00%		7.00%
Amortization term	25 years
Payment of principal balance						$ 66,500,000
Mortgage Financing - Wells Fargo Bank | One-month LIBOR
Debt
Debt, floating rate of interest basis	one-month LIBOR
Debt, floating rate of interest, basis spread (as a percent)	3.75%
Mortgage Financing
Debt, floating rate of interest basis	one-month LIBOR
Debt, floating rate of interest, basis spread (as a percent)	3.75%