Difference between Federal Income Tax Rate and Effective Income Tax Rate (Detail)		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Federal income tax rate		21.00%	21.00%	21.00%	35.00%
Foreign tax expense reduced by foreign tax credits		6.90%	0.40%	(0.30%)
Income subject to dividends-received deduction		(1.30%)	(0.30%)	(18.30%)
Tax-exempt interest		(6.00%)	(1.00%)	(36.00%)
State taxes, net of federal tax benefit		4.90%	0.90%	13.80%
Prior period adjustment		(2.60%)	0.20%	(0.10%)
Other, net	[1]	(3.40%)	(0.40%)	(18.10%)
Effective tax rate		19.50%	20.80%	(38.00%)

[1] For 2020, includes various general business credits and tax benefits resulting from corporate acquisitions and dispositions, partially offset by disallowed expense deductions primarily related to certain executive compensation paid in 2020.