Income Taxes - Differences Between 35 Percent Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 46	$ 88	$ 187	$ 239
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(9)	(8)	(27)	(25)
Dividend received exclusion	(8)	(8)	(25)	(24)
Other	(2)	1	(5)	3
Total provision for income taxes	$ 27	$ 73	$ 130	$ 193
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(7.00%)	(3.20%)	(5.10%)	(3.70%)
Dividend received exclusion	(6.20%)	(3.20%)	(4.70%)	(3.50%)
Other	(0.90%)	0.30%	(0.80%)	0.40%
Provision for income taxes	20.90%	28.90%	24.40%	28.20%