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, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 88	$ 85	$ 239	$ 233
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(8)	(9)	(25)	(25)
Dividend received exclusion	(8)	(8)	(24)	(23)
Other	1	1	3	2
Total provision for income taxes	$ 73	$ 69	$ 193	$ 187
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(3.20%)	(3.70%)	(3.70%)	(3.80%)
Dividend received exclusion	(3.20%)	(3.30%)	(3.50%)	(3.50%)
Other	0.30%	0.40%	0.40%	0.40%
Provision for income taxes	28.90%	28.40%	28.20%	28.10%