Income Taxes - Differences Between Statutory Income Tax Rate and Effective Income Tax Rate (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Increase (decrease) resulting from:
Tax at statutory rate	$ 183	$ 240	$ 344	$ (91)
Tax-exempt income from municipal bonds	(5)	(5)	(10)	(10)
Dividend received exclusion	(4)	(4)	(9)	(8)
Other	(5)	5	(8)	(5)
Total provision (benefit) for income taxes	$ 169	$ 236	$ 317	$ (114)
Increase (decrease) resulting from:
Tax at statutory rate	21.00%	21.00%	21.00%	21.00%
Effective Income Tax Rate Reconciliation, Tax Exempt Income, Percent	(0.60%)	(0.40%)	(0.60%)	2.30%
Effective Income Tax Rate Reconciliation, Deduction, Dividend, Percent	(0.50%)	(0.30%)	(0.50%)	1.90%
Other	(0.50%)	0.30%	(0.60%)	1.30%
Provision for income taxes	19.40%	20.60%	19.30%	26.50%