Convertible Senior Notes - Due 2022 (Details)	1 Months Ended	12 Months Ended
	Jan. 31, 2015 USD ($) day $ / shares	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	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 2022 [Member]
Debt Instrument [Line Items]
Principal	$ 400,000,000	399,997,000	400,000,000
Interest rate	2.50%
Proceeds from offering	$ 387,200,000
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%
Conversion ratio	0.0298806
Conversion price (usd per share) | $ / shares	$ 33.47
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	$ 88,900,000
Net deferred tax liabilities	31,800,000
Liability component
Principal	$ 400,000,000	399,997,000	400,000,000
Less: Debt discount, net		(62,747,000)	(75,754,000)
Net carrying amount		$ 337,250,000	324,246,000
Remaining contractual life		4 years
Interest expense
Contractual interest expense		$ 10,000,000	10,000,000	9,639,000
Amortization of debt discount		13,007,000	12,139,000	10,942,000
Total		$ 23,007,000	$ 22,139,000	$ 20,581,000
Effective interest rate of the liability component		6.50%	6.50%	0.00%
Senior Notes [Member] | Convertible Senior Notes Due 2022 [Member] | Level 2 [Member]
Liability component
Fair value of Notes		$ 417,600,000