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		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 64	$ 53	$ 191	$ 103
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(8)	(8)	(24)	(25)
Dividend received exclusion	(6)	(6)	(18)	(17)
Other	1	2	3	3
Total provision for income taxes	$ 51	$ 41	$ 152	$ 64
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(4.40%)	(5.30%)	(4.40%)	(8.50%)
Dividend received exclusion	(3.30%)	(4.00%)	(3.30%)	(5.80%)
Other	0.70%	1.30%	0.50%	1.10%
Provision (benefit) for income taxes	28.00%	27.00%	27.80%	21.80%