Convertible Debt - Interest Expense (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Contractual coupon interest	$ 6,282	$ 4,467	$ 4,025
Accretion of debt discount	15,931	8,800	6,552
Amortization of debt issuance costs	2,077	1,270	1,126
Total interest expense related to convertible notes	24,290	14,537	11,703
Amortization of debt discount and issuance costs	18,008	$ 10,068	$ 7,678
Total interest expense	24,290
2% Convertible Senior Notes
Debt Instrument [Line Items]
Contractual coupon interest	1,177
Amortization of debt discount and issuance costs	2,461
Total interest expense	3,638
1.25% Convertible Senior Notes
Debt Instrument [Line Items]
Contractual coupon interest	4,336
Amortization of debt discount and issuance costs	13,549
Total interest expense	17,885
1.375% Convertible Senior Notes
Debt Instrument [Line Items]
Contractual coupon interest	769
Amortization of debt discount and issuance costs	1,998
Total interest expense	$ 2,767