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, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 85	$ 91	$ 233	$ 170
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(9)	(9)	(25)	(25)
Dividend received exclusion	(8)	(7)	(23)	(21)
Other	1	1	2	3
Total provision for income taxes	$ 69	$ 76	$ 187	$ 127
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(3.70%)	(3.50%)	(3.80%)	(5.20%)
Dividend received exclusion	(3.30%)	(2.70%)	(3.50%)	(4.30%)
Other	0.40%	0.50%	0.40%	0.70%
Provision for income taxes	28.40%	29.30%	28.10%	26.20%