Income Taxes - Total Income Tax Expense (Benefit) and Resulting Effective Tax Rate (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income (loss) before income taxes times 21% (35% for 2017) statutory federal tax rate	$ 425	$ 463	$ 675
Amortization of tax-advantaged investments	132	127	104
Foreign tax adjustments	0	2	1
Tax-exempt interest income	(30)	(30)	(37)
Corporate-owned life insurance income	(29)	(29)	(46)
State income tax, net of federal tax benefit	31	34	19
Tax credits	(231)	(234)	(218)
Tax Cuts and Jobs Act	0	7	147
Other	16	4	(8)
Total income tax expense (benefit)	$ 314	$ 344	$ 637
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Federal income tax rate	21.00%	21.00%	35.00%
Amortization of tax-advantaged investments (as a percent)	6.50%	5.80%	5.40%
Foreign tax adjustments (as a percent)	0.00%	0.10%	0.10%
Tax-exempt interest income (as a percent)	(1.50%)	(1.40%)	(1.90%)
Corporate-owned life insurance income (as a percent)	(1.40%)	(1.30%)	(2.40%)
State income tax, net of federal tax benefit (as a percent)	1.50%	1.50%	1.00%
Tax credits (as a percent)	(11.40%)	(10.60%)	(11.30%)
Tax Cuts and Jobs Act (as a percent)	0	0.003	0.076
Other (as a percent)	0.90%	0.20%	(0.50%)
Total income tax expense (benefit) (as a percent)	15.60%	15.60%	33.00%