Income Taxes - Differences Between 35 Percent Statutory Income Tax Rate and Effective Income Tax Rate (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 61	$ 42
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(8)	(8)
Dividend received exclusion	(8)	(7)
Other	1	1
Total provision for income taxes	$ 46	$ 28
Tax at statutory rate	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(4.60%)	(6.70%)
Dividend received exclusion	(4.60%)	(5.90%)
Other	0.60%	1.10%
Provision for income taxes	26.40%	23.50%