Income Taxes - Differences Between 35 Percent Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Tax Disclosure [Abstract]
Tax at statutory rate	$ 87	$ 37	$ 148	$ 79
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(8)	(8)	(16)	(16)
Dividend received exclusion	(7)	(7)	(15)	(14)
Other	0	1	1	2
Total provision for income taxes	$ 72	$ 23	$ 118	$ 51
Tax at statutory rate	35.00%	35.00%	35.00%	35.00%
Increase (decrease) resulting from:
Tax-exempt income from municipal bonds	(3.20%)	(7.50%)	(3.80%)	(7.10%)
Dividend received exclusion	(2.80%)	(6.50%)	(3.60%)	(6.20%)
Other	0.00%	0.50%	0.40%	0.90%
Provision for income taxes	29.00%	21.50%	28.00%	22.60%