Earnings Per Share - Additional Information (Detail) - shares shares in Millions	3 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Feb. 22, 2018	Dec. 31, 2017	Dec. 31, 2016	Mar. 18, 2013
Earnings Per Share Disclosure [Line Items]
Anti-dilutive shares	0.2	0.4
Debt instrument, interest rate, stated percentage	5.75%
1.50% Senior Subordinated Notes Due March 15, 2018
Earnings Per Share Disclosure [Line Items]
Debt instrument, interest rate, stated percentage	1.50%
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 | 1.50% Senior Subordinated Notes Due March 15, 2018
Earnings Per Share Disclosure [Line Items]
Debt instrument, interest rate, stated percentage	1.50%	1.50%	1.50%	1.50%	1.50%	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%
Performance Shares
Earnings Per Share Disclosure [Line Items]
Anti-dilutive shares	0.1	0.2