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, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 44	$ 58	$ 141	$ 151
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(9)	(8)	(18)	(17)
Dividend received exclusion	(9)	(8)	(17)	(16)
Other	2	1	(3)	2
Total provision for income taxes	$ 28	$ 43	$ 103	$ 120
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(7.00%)	(4.80%)	(4.50%)	(3.90%)
Dividend received exclusion	(7.00%)	(4.80%)	(4.20%)	(3.70%)
Other	0.90%	0.50%	(0.80%)	0.40%
Provision for income taxes	21.90%	25.90%	25.50%	27.80%