Net Carrying Amount of Debt (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018	Feb. 22, 2018	Oct. 27, 2017	Sep. 30, 2017
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	5.75%
Principal amount of long term debt				$ 240.7
5.75% Senior Subordinated Notes Due August 2023
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	5.75%	5.75%
Convertible Notes
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	1.50%				1.50%
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	$ 94.4	$ 109.7	$ 125.0