Intangible Assets - Estimated Amortization Expense for VOBA in Future Fiscal Periods (Detail)	Sep. 30, 2018 USD ($)
Goodwill and Intangible Assets Disclosure [Abstract]
2018	$ 15,000,000
2019	69,000,000
2020	89,000,000
2021	85,000,000
2022	79,000,000
Thereafter	$ 438,000,000