Income Taxes - Differences Between 35 Percent Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 58	$ 87	$ 151	$ 148
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(8)	(8)	(17)	(16)
Dividend received exclusion	(8)	(7)	(16)	(15)
Other	1	0	2	1
Total provision for income taxes	$ 43	$ 72	$ 120	$ 118
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(4.80%)	(3.20%)	(3.90%)	(3.80%)
Dividend received exclusion	(4.80%)	(2.80%)	(3.70%)	(3.60%)
Other	0.50%	0.00%	0.40%	0.40%
Provision for income taxes	25.90%	29.00%	27.80%	28.00%