Debt (Convertible Debt) (Details) $ / shares in Units, $ in Thousands			12 Months Ended
	Nov. 13, 2017 USD ($)	May 12, 2017 USD ($) $ / shares shares	Dec. 31, 2018 USD ($) Integer shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 24, 2017 USD ($)	Nov. 30, 2010 USD ($)
Debt Instrument
Proceeds from convertible debt			$ 0	$ 200,000	$ 0
Purchase of call options related to convertible notes		$ 20,500
Convertible Debt [Abstract]
Capped call transaction cap price (in dollars per share) | $ / shares		$ 88.7570
Effective interest rate						5.00%
Amortization of debt issuance costs			1,806	2,014	1,536
Interest Expense, Debt [Abstract]
Accretion of convertible notes discount			2,596	3,410	2,144
Proceeds from bond hedge settlement convertible notes			0	58,564	0
Gain (Loss) on Extinguishment of Debt			0	(137)	0
2024 Notes
Debt Instrument
Convertible notes face amount			$ 200,000	200,000		$ 200,000
Amount before overallotment		$ 175,000
Debt Instrument face amount overallotment						25,000
Annual principal interest rate		3.25%
Deferred finance costs, gross				(7,400)
Convertible Debt [Abstract]
Conversion rate		15.3227
Conversion price (in dollars per share) | $ / shares		$ 65.2626
Contingent conversion price (in dollars per shares) | $ / shares		$ 84.84
Aggregate shares to be issued upon conversion, convertible (in shares) | shares		3,064,540
Convertible debt stock price threshold trigger percent			130.00%
Convertible debt trading days threshold | Integer			20
Convertible, threshold consecutive trading days | Integer			30
Percentage of average of closing price of common stock			98.00%
Carrying value of liability			$ 183,749	181,153		179,500
Equity component			20,459	20,459		20,500
Debt issuance costs, recorded to APIC						700
Debt issuance costs, recorded as contra-debt						$ 6,700
Amortization of debt issuance costs			800	500
Unamortized discount			16,251	18,847
Interest Expense, Debt [Abstract]
Contractual coupon rate of interest			6,500	4,207	0
Accretion of convertible notes discount			2,596	1,612	0
Interest expense			$ 9,096	5,819	0
2017 Notes
Debt Instrument
Convertible notes face amount							$ 115,000
Debt Instrument, Repurchased Face Amount	$ 11,500	$ 103,500
Options issued in debt conversion (in shares) | shares		624,044	114,778
Annual principal interest rate							3.25%
Convertible Debt [Abstract]
Carrying value of liability		$ 113,943
Effective interest rate		3.00%
Interest Expense, Debt [Abstract]
Contractual coupon rate of interest			$ 0	3,270	3,738
Accretion of convertible notes discount		$ 1,000	0	1,797	2,144
Interest expense			0	5,067	5,882
Repurchase of debt fair value equity component		60,000
Convertible debt repurchased carrying value		102,548
Repurchase of debt fair value debt component		103,637
Gain loss) on extinguishment of debt, before write off of debt issuance cost				(1,089)
Write off of Deferred Debt Issuance Cost				297
Proceeds from bond hedge settlement convertible notes		$ 58,600
Percent of repurchase principal	10.00%	90.00%
Debt Instrument, Repurchase Amount		$ 165,300
Gain (Loss) on Extinguishment of Debt				(1,386)
Credit Agreement 2015 | Revolving Credit Facility
Convertible Debt [Abstract]
Amortization of debt issuance costs			$ 1,000	$ 1,000	$ 1,000