Convertible Senior Notes - Due 2017 (Details)		1 Months Ended		3 Months Ended		6 Months Ended
	Jun. 01, 2017 USD ($) shares	Jun. 30, 2016 USD ($)	Jun. 30, 2012 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Debt Instrument [Line Items]
Repayments of convertible senior notes						$ 54,997,000	$ 323,225,000
Extinguishment of debt				$ 0	$ 5,380,000	0	5,380,000
Interest expense
Amortization of debt discount						13,822,000	12,770,000
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		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				126,000	778,000	315,000	1,723,000
Amortization of debt discount				540,000	2,752,000	1,251,000	6,078,000
Total				$ 666,000	$ 3,530,000	$ 1,566,000	$ 7,801,000
Effective interest rate of the liability component		6.02%		6.02%	6.02%	6.02%	6.02%