Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Tax provision at U.S. federal statutory rate	$ 111	$ 138	$ 308	$ 396
Tax-exempt interest	(12)	(14)	(36)	(43)
Effective Income Tax Rate Reconciliation, Deduction, Dividends, Amount	4	3	5	5
Executive compensation	1	0	6	5
Increase in deferred tax valuation allowance	(6)	0	(19)	0
Stock-based compensation	0	3	1	7
Effective Income Tax Rate Reconciliation, Disposition of Business, Amount	(8)	0	(8)	0
Effective Income Tax Rate Reconciliation, Tax Credit, Amount	(4)	0	(4)	0
Effective Income Tax Rate Reconciliation, Prior Year Income Taxes, Amount	11	0	11	0
Other	6	0	0	(1)
Income tax expense	$ 73	$ 118	$ 268	$ 347