Significant Accounting Policies (Details) - USD ($)	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2015
Debt Instrument [Line Items]
Cash and cash equivalents	$ 436,715,000	$ 430,196,000	$ 541,835,000	$ 373,173,000
Reimbursement revenue	2,600,000	$ 6,300,000
Convertible Senior Notes Due 2022 | Senior Notes
Debt Instrument [Line Items]
Aggregate principal	400,000,000		400,000,000		$ 400,000,000
Interest rate					2.50%
Carrying amount, liability component	327,432,000		$ 324,246,000
Carrying amount, equity component	88,900,000
Accounting Standards Update 2016-09
Debt Instrument [Line Items]
Excess tax benefits related to share-based compensation	$ 4,200,000