Income Taxes - Total Income Tax Expense (Benefit) and Resulting Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income (loss) before income taxes times 21% (35% for 2017 and 2016) statutory federal tax rate	$ 463	$ 675	$ 339
Amortization of tax-advantaged investments	127	104	88
Foreign tax adjustments	2	1	1
Tax-exempt interest income	(30)	(37)	(25)
Corporate-owned life insurance income	(29)	(46)	(44)
State income tax, net of federal tax benefit	34	19	11
Tax credits	(234)	(218)	(208)
Tax Cuts and Jobs Act	7	147	0
Other	4	(8)	17
Total income tax expense (benefit)	$ 344	$ 637	$ 179
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal income tax rate	21.00%	35.00%	35.00%
Amortization of tax-advantaged investments (as a percent)	5.80%	5.40%	9.00%
Foreign tax adjustments (as a percent)	0.10%	0.10%	0.10%
Tax-exempt interest income (as a percent)	(1.40%)	(1.90%)	(2.60%)
Corporate-owned life insurance income (as a percent)	(1.30%)	(2.40%)	(4.50%)
State income tax, net of federal tax benefit (as a percent)	1.50%	1.00%	1.10%
Tax credits (as a percent)	(10.60%)	(11.30%)	(21.30%)
Tax Cuts and Jobs Act (as a percent)	0.30%	7.60%	0.00%
Other (as a percent)	0.20%	(0.50%)	1.70%
Total income tax expense (benefit) (as a percent)	15.60%	33.00%	18.50%