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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 91	$ 64	$ 170	$ 191
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(9)	(8)	(25)	(24)
Dividend received exclusion	(7)	(6)	(21)	(18)
Other	1	1	3	3
Total provision for income taxes	$ 76	$ 51	$ 127	$ 152
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(3.50%)	(4.40%)	(5.20%)	(4.40%)
Dividend received exclusion	(2.70%)	(3.30%)	(4.30%)	(3.30%)
Other	0.50%	0.70%	0.70%	0.50%
Provision for income taxes	29.30%	28.00%	26.20%	27.80%