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		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 52	$ 10	$ 128	$ 49
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(9)	(8)	(16)	(17)
Dividend received exclusion	(6)	(6)	(12)	(11)
Other	1	1	1	2
Total provision (benefit) for income taxes	$ 38	$ (3)	$ 101	$ 23
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(6.10%)	(27.60%)	(4.40%)	(12.10%)
Dividend received exclusion	(4.20%)	(20.70%)	(3.30%)	(7.80%)
Other	1.00%	3.00%	0.40%	1.20%
Provision (benefit) for income taxes	25.70%	(10.30%)	27.70%	16.30%