Income Taxes - Differences Between Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 56	$ 44	$ 45	$ 141
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(5)	(9)	(10)	(18)
Dividend received exclusion	(5)	(9)	(8)	(17)
Other	1	2	1	(3)
Total provision for income taxes	$ 47	$ 28	$ 28	$ 103
Tax at statutory rate	21.00%	35.00%	21.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(1.90%)	(7.00%)	(4.70%)	(4.50%)
Dividend received exclusion	(1.50%)	(7.00%)	(3.70%)	(4.20%)
Other	0.20%	0.90%	0.50%	(0.80%)
Provision for income taxes	17.80%	21.90%	13.10%	25.50%