Income Taxes - Differences Between 35 Percent Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 93	$ 61
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(9)	(8)
Dividend received exclusion	(8)	(8)
Other	1	1
Total provision for income taxes	$ 77	$ 46
Tax at statutory rate	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(3.40%)	(4.60%)
Dividend received exclusion	(3.00%)	(4.60%)
Other	0.50%	0.60%
Provision for income taxes	29.10%	26.40%