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, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Increase (decrease) resulting from:
Tax at statutory rate	$ 111	$ 56	$ 293	$ 45
Tax-exempt income from municipal bonds	(4)	(5)	(9)	(10)
Dividend received exclusion	(4)	(5)	(8)	(8)
Other	(1)	1	(2)	1
Total provision for income taxes	$ 102	$ 47	$ 274	$ 28
Increase (decrease) resulting from:
Tax at statutory rate	21.00%	21.00%	21.00%	21.00%
Tax-exempt income from municipal bonds	(0.80%)	(1.90%)	(0.60%)	(4.70%)
Dividend received exclusion	(0.80%)	(1.50%)	(0.60%)	(3.70%)
Other	(0.20%)	0.20%	(0.20%)	0.50%
Provision for income taxes	19.20%	17.80%	19.60%	13.10%