Convertible Senior Notes - Due 2017 (Details)		1 Months Ended		3 Months Ended			9 Months Ended
	Jun. 01, 2017 USD ($) shares	Jun. 30, 2016 USD ($)	Jun. 30, 2012 USD ($) $ / shares	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Jun. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Repayments of convertible senior notes							$ 55,000,000	$ 323,225,000
Extinguishment of debt				$ 0	$ 0		0	5,380,000
Interest expense
Amortization of debt discount							20,325,000	19,392,000
Senior Notes | Convertible Senior Notes Due 2017
Debt Instrument [Line Items]
Principal			$ 275,000,000	0			0		$ 55,000,000
Interest rate			1.375%
Proceeds from offering			$ 266,200,000
Conversion ratio			0.0358038
Conversion price (USD per share) | $ / shares			$ 27.93
Repayments of convertible senior notes	$ 55,400,000	$ 323,200,000
Debt, aggregate principal amount repurchased		220,000,000				$ 220,000,000
Cash receipt for settlement of outstanding bond hedges and warrants		$ 12,600,000
Extinguishment of debt						5,400,000
Reduction of additional paid-in capital for difference between consideration transferred and fair value of liability component						$ 108,700,000
Issuance of stock upon note conversion (in shares) | shares	819,901
Liability component
Principal			$ 275,000,000	0			0		55,000,000
Less: Debt discount, net				0			0		(1,251,000)
Net carrying amount				0			0		$ 53,749,000
Interest expense
Contractual interest expense				0	189,000		315,000	1,912,000
Amortization of debt discount				0	621,000		1,251,000	6,699,000
Total				$ 0	$ 810,000		$ 1,566,000	$ 8,611,000
Effective interest rate of the liability component				6.02%	6.02%		6.02%	6.02%