Schedule of Impact on Earnings Per Share Calculation (Parenthetical) (Detail)	Sep. 30, 2018	Feb. 22, 2018	Sep. 30, 2017
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%
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%