Income Taxes - Effective Income Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Income Tax Disclosure [Abstract]
Tax provision at U.S. federal statutory rate	$ 63	$ 72
Tax-exempt interest	(11)	(12)
Effective Income Tax Rate Reconciliation, Nondeductible Expense, Share-based Payment Arrangement, Amount	5	5
Increase in deferred tax valuation allowance	(1)	(6)
Other	4	0
Income tax expense	$ 54	$ 71