Net Carrying Amount of Debt (Parenthetical) (Detail) - USD ($)	Dec. 31, 2018	Feb. 22, 2018	Dec. 31, 2017	Oct. 27, 2017	Jun. 30, 2017	Dec. 31, 2016	Mar. 18, 2013
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	5.75%
Principal amount of long term debt				$ 240,700,000	$ 0
Convertible Notes
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	1.50%
Convertible Notes | 1.50% Senior Subordinated Notes Due March 15, 2018
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	1.50%	1.50%	1.50%			1.50%	1.50%
Principal amount of long term debt		$ 125,000,000					$ 400,000,000
Convertible Notes | 5.75% Senior Subordinated Notes Due August 2023
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	5.75%	5.75%	5.75%
Principal amount of long term debt	$ 109,700,000	$ 125,000,000