Income Taxes - Total Income Tax Expense (Benefit) and Resulting Effective Tax Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Income (loss) before income taxes times 35% statutory federal tax rate	$ 339	$ 428	$ 445
Amortization of tax-advantaged investments	88	81	69
Foreign tax adjustments	1	(1)	10
Tax-exempt interest income	(25)	(18)	(16)
Corporate-owned life insurance income	(44)	(45)	(41)
State income tax, net of federal tax benefit	11	22	15
Tax credits	(208)	(155)	(134)
Other	17	(9)	(22)
Total income tax expense (benefit)	$ 179	$ 303	$ 326
Federal income tax rate	35.00%	35.00%	35.00%
Amortization of tax-advantaged investments (as a percent)	9.00%	6.70%	5.40%
Foreign tax adjustments (as a percent)	0.10%	(0.10%)	0.80%
Tax-exempt interest income (as a percent)	(2.60%)	(1.50%)	(1.30%)
Corporate-owned life insurance income (as a percent)	(4.50%)	(3.60%)	(3.20%)
State income tax, net of federal tax benefit (as a percent)	1.10%	1.80%	1.10%
Tax credits (as a percent)	(21.30%)	(12.70%)	(10.50%)
Other (as a percent)	1.70%	(0.80%)	(1.70%)
Total income tax expense (benefit) (as a percent)	18.50%	24.80%	25.60%