Income Taxes - Differences Between 35 Percent Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 97	$ 93
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(9)	(9)
Dividend received exclusion	(8)	(8)
Other	(5)	1
Total provision for income taxes	$ 75	$ 77
Tax at statutory rate	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(3.30%)	(3.40%)
Dividend received exclusion	(2.90%)	(3.00%)
Other	(1.60%)	0.50%
Provision for income taxes	27.20%	29.10%