Income Taxes - Differences Between Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 130	$ 46	$ 175	$ 187
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(5)	(9)	(15)	(27)
Dividend received exclusion	(3)	(8)	(11)	(25)
Impact of tax reform	(50)	0	(50)	0
Other	(7)	(2)	(6)	(5)
Total provision for income taxes	$ 65	$ 27	$ 93	$ 130
Tax at statutory rate	21.00%	35.00%	21.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(0.80%)	(7.00%)	(1.80%)	(5.10%)
Dividend received exclusion	(0.50%)	(6.20%)	(1.30%)	(4.70%)
Impact of tax reform	(8.10%)	0.00%	(6.00%)	0.00%
Other	(1.10%)	(0.90%)	(0.70%)	(0.80%)
Provision for income taxes	10.50%	20.90%	11.20%	24.40%