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, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Increase (decrease) resulting from:
Tax at statutory rate	$ 240	$ 111	$ (91)	$ 293
Tax-exempt income from municipal bonds	(5)	(4)	(10)	(9)
Dividend received exclusion	(4)	(4)	(8)	(8)
Other	5	(1)	(5)	(2)
Total provision (benefit) for income taxes	$ 236	$ 102	$ (114)	$ 274
Increase (decrease) resulting from:
Tax at statutory rate	21.00%	21.00%	21.00%	21.00%
Tax-exempt income from municipal bonds	(0.40%)	(0.80%)	2.30%	(0.60%)
Dividend received exclusion	(0.30%)	(0.80%)	1.90%	(0.60%)
Other	0.30%	(0.20%)	1.30%	(0.20%)
Provision for income taxes	20.60%	19.20%	26.50%	19.60%