Schedule of Impact on Earnings Per Share Calculation (Parenthetical) (Detail)	Mar. 31, 2019	Mar. 14, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Feb. 22, 2018
Earnings Per Share Disclosure [Line Items]
Debt instrument, interest rate, stated percentage	5.75%
5.75% Senior Subordinated Notes Due August 2023
Earnings Per Share Disclosure [Line Items]
Debt instrument, interest rate, stated percentage	5.75%	5.75%			5.75%
Convertible Notes
Earnings Per Share Disclosure [Line Items]
Debt instrument, interest rate, stated percentage				1.50%
Convertible Notes | 5.75% Senior Subordinated Notes Due August 2023
Earnings Per Share Disclosure [Line Items]
Debt instrument, interest rate, stated percentage	5.75%		5.75%		5.75%	5.75%