Income Taxes - Total Income Tax Expense (Benefit) and Resulting Effective Tax Rate (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income (loss) before income taxes times 35% statutory federal tax rate	$ 675	$ 339	$ 428
Amortization of tax-advantaged investments	104	88	81
Foreign tax adjustments	1	1	(1)
Tax-exempt interest income	(37)	(25)	(18)
Corporate-owned life insurance income	(46)	(44)	(45)
State income tax, net of federal tax benefit	19	11	22
Tax credits	(218)	(208)	(155)
Tax Cuts and Jobs Act	147	0	0
Other	(8)	17	(9)
Total income tax expense (benefit)	$ 637	$ 179	$ 303
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal income tax rate	35.00%	35.00%	35.00%
Amortization of tax-advantaged investments (as a percent)	5.40%	9.00%	6.70%
Foreign tax adjustments (as a percent)	0.10%	0.10%	(0.10%)
Tax-exempt interest income (as a percent)	(1.90%)	(2.60%)	(1.50%)
Corporate-owned life insurance income (as a percent)	(2.40%)	(4.50%)	(3.60%)
State income tax, net of federal tax benefit (as a percent)	1.00%	1.10%	1.80%
Tax credits (as a percent)	(11.30%)	(21.30%)	(12.70%)
Tax Cuts and Jobs Act (as a percent)	7.60%	0.00%	0.00%
Other (as a percent)	(0.50%)	1.70%	(0.80%)
Total income tax expense (benefit) (as a percent)	33.00%	18.50%	24.80%