Interest Expense Related to Cash Convertible Senior Notes (Detail) - 1.25% Cash Convertible Senior Notes [Member] - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Coupon interest at 1.25%	$ 4,312	$ 4,312	$ 4,312
Amortization of discounts and debt issuance costs	13,208	12,585	11,994
Total interest expense related to the 1.25% Notes	$ 17,520	$ 16,897	$ 16,306