Income Taxes - Summary of Income Tax Expense Allocated Based on Separate Tax Return Basis (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Current income taxes: Federal taxes	$ 47	$ 41	$ 26
Current income taxes: State taxes	4	6	2
Total current income taxes	51	47	28
Deferred income taxes: Federal taxes	30	32	128
Deferred income taxes: State taxes	3	0	1
Total deferred income taxes	33	32	129
Total income taxes	$ 84	$ 79	$ 157