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, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 37	$ 52	$ 79	$ 128
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(8)	(9)	(16)	(16)
Dividend received exclusion	(7)	(6)	(14)	(12)
Other	1	1	2	1
Total provision for income taxes	$ 23	$ 38	$ 51	$ 101
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(7.50%)	(6.10%)	(7.10%)	(4.40%)
Dividend received exclusion	(6.50%)	(4.20%)	(6.20%)	(3.30%)
Other	0.50%	1.00%	0.90%	0.40%
Provision for income taxes	21.50%	25.70%	22.60%	27.70%