Debt (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Oct. 31, 2011	Dec. 31, 2010
Interest expense
Interest expense		$ 4,735	$ 17,592	$ 18,117
Less: interest capitalized				(59)
Total interest expense		4,735	$ 17,592	$ 18,058
Credit Agreement 2016
Debt
Maximum borrowing capacity		$ 1,000,000
Debt, maturity term		5 years
Period of extension in maturity		1 year
Mortgage Financing
Mortgage term		5 years
Revolving line of credit
Debt
Maximum borrowing capacity		$ 75,000
Unused commitment fee (as a percent)		0.06%
Outstanding amount		$ 0
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
Convertible Senior Notes, stated percentage	1.00%	1.00%	1.00%	1.00%	1.00%	1.00%
Conversion price (in dollars per share)		$ 156.61				$ 47.69
Convertible debt, number of shares into which convertible						5,200,000
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		$ 12,800
Redemption price of convertible notes as a percentage of principal amount, due to change in control		100.00%
Equity component		$ 57,900
Amortization schedule		5 years
Effective interest rate (as a percent)		6.70%
Interest expense incurred in connection with the entity's convertible notes
Contractual coupon rate of interest		$ 404	$ 2,151	$ 2,500
Discount amortization		3,020	11,057	11,178
Interest expense-convertible notes		3,424	13,208	$ 13,678
Carrying amount related to convertible notes
Principal balance	$ 138,750	5,600	138,750
Convertible debt, discount to reduce net carrying value	(12,336)	(213)	(12,336)
Discount, accumulated amortization	19,819	1,085	19,819
Carrying amount	126,414	$ 5,387	126,414
Shares issued for conversion of convertible notes (in shares)		2,000,000
Value of convertible note settled due to conversion requests		$ 133,200
Extinguishment loss		$ (3,700)
Number of common shares bought under note hedge		2,000,000
Number of shares that could be issued for future conversions of outstanding convertible notes (in shares)		117,000
Mortgage Financing
Effective interest rate (as a percent)		6.70%
Future maturities relating to mortgage financings
Carrying amount	126,414	$ 5,387	$ 126,414
Mortgage Financing - Wells Fargo Bank
Mortgage Financing
Mortgage loan under credit agreement							$ 70,000
Payment of principal balance	$ 66,500