Convertible Senior Notes (Carrying Amounts of Liability and Equity Components) (Details) - Convertible Senior Notes [Member] - USD ($)		Sep. 30, 2018		Aug. 31, 2018	Aug. 24, 2018	Dec. 31, 2017	Jun. 30, 2014
2019 Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Amount of the equity component	[1]	$ 9,196,000				$ 45,808,000
Convertible senior notes		66,138,000				329,972,000	$ 330,000,000
Unamortized debt discount	[2]	(1,496,000)				(15,469,000)
Unamortized transaction costs related to the debt component		(260,000)				(2,509,000)
Contractual coupon interest accrual		5,074,000				7,425,000
Contractual coupon interest payment		(4,702,000)				(7,425,000)
Net carrying amount		64,754,000				$ 311,994,000
2028 Convertible Senior Notes [Member]
Debt Instrument [Line Items]
Amount of the equity component		327,305,000	[3]		$ 333,468,000
Convertible senior notes		880,000,000		$ 880,000,000
Unamortized debt discount	[4]	(331,547,000)
Unamortized transaction costs related to the debt component		(10,065,000)
Contractual coupon interest accrual		1,467,000
Net carrying amount		$ 539,855,000

[1] Net of $236 thousands of transaction costs related to the equity component of the 2019 Notes.

[2] As of September 30, 2018, the remaining period over which the unamortized debt discount will be amortized is 0.8 years.

[3] Net of $6,163 thousands of transaction costs related to the equity component of the 2028 Notes.

[4] As of September 30, 2018, the remaining period over which the unamortized debt discount will be amortized is 10 years.