Convertible Senior Notes - Due 2017 (Details) - USD ($)		1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 01, 2017	Jun. 30, 2016	Jun. 30, 2012	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Debt Instrument [Line Items]
Repayments of convertible senior notes						$ 0	$ 54,997,000
Interest expense
Amortization of debt discount						13,531,000	13,822,000
Senior Notes | Convertible Senior Notes Due 2017 [Member]
Debt Instrument [Line Items]
Principal			$ 275,000,000
Interest rate			1.375%
Proceeds from offering			$ 266,200,000
Repayments of convertible senior notes	$ 55,400,000	$ 323,200,000
Aggregate principal amount repurchased		220,000,000
Net cash receipt		$ 12,600,000
Debt conversion, converted instrument, shares issued (in shares)	819,901
Interest expense
Contractual interest expense				$ 0	$ 126,000	0	315,000
Amortization of debt discount				0	540,000	0	1,251,000
Total				$ 0	$ 666,000	$ 0	$ 1,566,000
Effective interest rate of the liability component				0.00%	6.02%	0.00%	6.02%