Schedule of Debt Obligations (Parenthetical) (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Mar. 16, 2018	Feb. 22, 2018	Dec. 31, 2017	Oct. 27, 2017	Jun. 30, 2017	Mar. 31, 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					$ 240.7	$ 0.0
1.50% Senior Subordinated Notes Due March 15, 2018
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	1.50%
5.75% Senior Subordinated Notes Due August 2023
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	5.75%
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%	1.50%
Principal amount of long term debt			$ 125.0						$ 400.0
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	$ 116.2	$ 8.8	$ 125.0