Convertible Senior Notes - Due 2023 (Details)	1 Months Ended	12 Months Ended
	Jun. 30, 2016 USD ($) day $ / shares	Dec. 31, 2017 USD ($) $ / shares	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Debt Instrument [Line Items]
Net deferred tax liabilities		$ 0	$ 89,992,000
Interest expense
Amortization of debt discount		26,868,000	26,182,000	$ 23,676,000
Senior Notes [Member] | Convertible Senior Notes Due 2023 [Member]
Debt Instrument [Line Items]
Principal	$ 402,500,000	402,500,000	402,500,000
Interest rate	2.75%
Proceeds from offering	$ 390,800,000
Trading period | 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 period | 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 of equity component	$ 101,000,000
Net deferred tax liabilities	33,500,000
Liability component
Principal	402,500,000	402,500,000	402,500,000
Less: Debt discount, net		(90,552,000)	(103,162,000)
Net carrying amount		$ 311,948,000	299,338,000
Remaining contractual life		5 years 6 months
Interest expense
Contractual interest expense		$ 11,060,000	6,158,000	0
Amortization of debt discount		12,610,000	6,648,000	0
Total		$ 23,670,000	$ 12,806,000	$ 0
Effective interest rate of the liability component		7.50%	7.50%	0.00%
Senior Notes [Member] | Convertible Senior Notes Due 2023 [Member] | Capped call [Member]
Interest expense
Payment of cost of capped call transactions	$ 33,900,000
Cap price (usd per share) | $ / shares		$ 64.68
Senior Notes [Member] | Convertible Senior Notes Due 2023 [Member] | Level 2 [Member]
Liability component
Fair value of Notes		$ 377,100,000