Debt Convertible Senior Notes (Details) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017 USD ($)	Dec. 31, 2017 USD ($) d	Dec. 31, 2016 USD ($) $ / shares	Dec. 31, 2015 USD ($)	Dec. 08, 2014 USD ($)
Debt Instrument [Line Items]
Convertible Senior Notes, if-converted value excess	$ 1,500	$ 1,500
Interest expense, Convertible Senior Notes, Amortization			$ 5,400
Interest Expense, Debt	3,000	11,000	10,900	$ 10,900
Proceeds from issuance of Term Loan		245,000	0	0
Repurchase of Convertible Debt		(547)
Repayments of Convertible Debt		0	4,459	0
Gain on extinguishment of Convertible Senior Notes		0	(421)	0
3.25% Convertible Senior Notes due 2020 [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Convertible Senior Notes, face amount			$ 172,500		$ 172,500
Convertible Senior Notes, interest rate, stated percentage			3.25%
Debt issuance cost			$ 5,100
Deferred financing costs			$ 4,200
Remaining amortization period of discount on the liability component			6 years
Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs		900
Debt Instrument, Unamortized Discount	15,356	$ 15,356	$ 20,221
Convertible Senior Notes, conversion ratio		31.1891
Convertible Senior Notes, conversion price (in dollars per share) | $ / shares			$ 32.06
Convertible Senior Notes, threshold trading days | d		20
Convertible Senior Notes, threshold consecutive trading days | d		30
Convertible Senior Notes, threshold percentage of stock price trigger		130.00%
Convertible Senior Notes, threshold consecutive business days		5 years
Debt Instrument, Convertible, Threshold Consecutive Trading Days, Following Consecutive Business Days		5 years
Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger, Following Consecutive Business Days		98.00%
Convertible Senior Notes, observation period		25 years
Carrying value of equity component, net of issuance costs	$ 29,211	$ 29,211	$ 29,211		30,700
Interest expense, Convertible Senior Notes, Amortization		5,500		5,300
Interest expense, Convertible Senior Notes		$ 5,400	$ 5,500	$ 5,600
Debt Conversion, Converted Instrument, Rate		100.00%
Debt Instrument, Interest Rate, Effective Percentage	6.90%	6.90%	6.60%
Convertible Senior Notes, fair value disclosures					$ 141,900
Proceeds from issuance of Term Loan		$ 172,500
Fair value of outstanding Convertible Senior Notes	$ 257,245	257,245	$ 165,223
Repurchase of Convertible Debt		5,200
Repayments of Convertible Debt		4,500
Repayments of Debt		4,400
Adjustments to Additional Paid in Capital, Equity Component of Convertible Debt, Subsequent Adjustments		500
Gain on extinguishment of Convertible Senior Notes		(400)
3.25% Convertible Senior Notes due 2020 [Member] | Other Current Assets [Member] | Convertible Debt [Member]
Debt Instrument [Line Items]
Deferred financing costs	$ 2,090	$ 2,090	$ 2,753