Convertible Senior Notes (Parenthetical) (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2014	Jul. 31, 2013
Debt Instrument [Line Items]
Long term debt, principal amount	$ 157,668	$ 156,647
4.25% Convertible Senior Notes (due 2018)
Debt Instrument [Line Items]
Debt instrument, maturity year	2018	2018
Debt instrument, interest rate	4.25%	4.25%		4.25%
Long term debt, principal amount	$ 21,178	$ 21,178		$ 100,000
3.25% Convertible Senior Notes (due 2020)
Debt Instrument [Line Items]
Debt instrument, maturity year	2020	2020
Debt instrument, interest rate	3.25%	3.25%
Long term debt, principal amount	$ 23,490	$ 22,469
4.875% Convertible Senior Notes (due 2020)
Debt Instrument [Line Items]
Debt instrument, maturity year	2020	2020
Debt instrument, interest rate	4.875%	4.875%	4.875%
Long term debt, principal amount	$ 113,000	$ 113,000	$ 115,000