Long-term Debt Senior Convertible Notes (Details) - USD ($)		12 Months Ended
	Aug. 12, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Senior Convertible Notes [Line Items]
Senior Convertible Notes, net carrying amount		$ 139,107,000	$ 130,856,000
Capped call transaction costs			24,195,000
Capped call, cap price		$ 60.00
Capitalized interest costs		$ 12,600,000	17,000,000	$ 25,100,000
1.50% Senior Convertible Notes Due 2021 [Member]
Senior Convertible Notes [Line Items]
Senior Convertible Notes, face amount		$ 172,500,000	172,500,000
Senior Convertible Notes, interest rate, stated percentage		1.50%
Net proceeds from Senior Convertible Notes			166,600,000
Senior Convertible Notes debt issuance costs, gross			5,900,000
Senior Convertible Notes, threshold trading days		20
Senior Convertible Notes, threshold consecutive trading days		30
Senior Convertible Notes, threshold percentage of stock price trigger		130.00%
Senior Convertible Notes, threshold business days, trading price trigger		5 days
Senior Convertible Notes, threshold trading days, trading price trigger		5 days
Senior Convertible Notes, principal amount of note		$ 1,000
Senior Convertible Notes, threshold percentage of trading price trigger		98.00%
Senior Convertible Notes, conversion rate		24.6914
Senior Convertible Notes, conversion price		$ 40.50
Senior Convertible Notes, initial fair value of liability component	$ 132,300,000
Senior Convertible Notes, effective interest rate	7.25%
Senior Convertible Notes, initial value of equity component	$ 40,217,000	$ 40,217,000	40,217,000
Senior Convertible Notes, interest expense		9,900,000	3,700,000
Senior Convertible Notes, unamortized discount		(30,183,000)	(37,513,000)
Unamortized deferred financing costs		3,210,000	4,131,000
Senior Convertible Notes, net carrying amount		139,107,000	130,856,000
Senior Convertible Notes, related issuance costs, equity component		(1,375,000)	(1,375,000)
Senior Convertible Notes, deferred tax liability, equity component		5,267,000	5,267,000
Senior Convertible Notes, net carrying amount, equity component		$ 33,575,000	$ 33,575,000