Interest Expense Related to 1.25% Notes (Detail) - 1.25% Notes [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Debt Instrument [Line Items]
Coupon interest at 1.25%	$ 1,078	$ 1,078	$ 3,234	$ 3,234
Amortization of discounts and debt issuance costs	3,685	3,509	10,876	10,358
Total interest expense related to the 1.25% Notes	$ 4,763	$ 4,587	$ 14,110	$ 13,592