Convertible Senior Notes - Due 2023 (Details)	1 Months Ended	3 Months Ended		6 Months Ended
	Jun. 30, 2016 USD ($) day $ / shares	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($) $ / shares	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Interest expense
Amortization of debt discount				$ 13,531,000	$ 13,822,000
Convertible Senior Notes Due 2023 | Senior Notes
Debt Instrument [Line Items]
Principal	$ 402,500,000	$ 402,500,000		402,500,000		$ 402,500,000
Interest rate	2.75%
Proceeds from offering	$ 390,800,000
Trading days | day	20
Redemption consecutive trading period	30 days
Redemption stock price conversion threshold (greater than or equal to)	130.00%
Consecutive measurement period	5 days
Percent of trading price (less than)	98.00%
Consecutive trading days | day	50
Conversion ratio	0.0204198
Conversion price (USD per share) | $ / shares	$ 48.97
Redemption trading period	19 days
Percent of principal amount plus accrued and unpaid interest	100.00%
Debt default principal amount percentage	25.00%
Debt instrument, term	7 years
Carrying amount, equity component	$ 101,000,000
Net deferred tax liabilities	33,500,000
Liability component
Principal	402,500,000	402,500,000		402,500,000		402,500,000
Less: Debt discount, net		(83,869,000)		(83,869,000)		(90,552,000)
Net carrying amount		318,631,000		$ 318,631,000		$ 311,948,000
Remaining contractual life				5 years
Interest expense
Contractual interest expense		2,767,000	$ 2,767,000	$ 5,534,000	5,526,000
Amortization of debt discount		3,351,000	3,104,000	6,682,000	6,179,000
Total		$ 6,118,000	$ 5,871,000	$ 12,216,000	$ 11,705,000
Effective interest rate of the liability component		7.50%	7.50%	7.50%	7.50%
Convertible Senior Notes Due 2023 | Senior Notes | Call Option
Interest expense
Payment of cost of capped call transactions	$ 33,900,000
Cap price (USD per share) | $ / shares				$ 64.68
Convertible Senior Notes Due 2023 | Senior Notes | Significant Other Observable Inputs (Level 2)
Liability component
Fair value of notes		$ 401,500,000		$ 401,500,000