Difference between Federal Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of Statutory Federal Tax Rate [Line Items]
Federal income tax rate	35.00%	35.00%	35.00%
Foreign tax credits	(6.40%)	(0.60%)	(0.40%)
Income subject to dividends-received deduction	(25.50%)	(1.60%)	(1.60%)
Tax-exempt interest	(94.10%)	(6.50%)	(6.80%)
State taxes, net of federal tax benefit	6.50%	0.60%	0.20%
Prior period adjustment	(3.30%)	2.40%	(0.20%)
Tax benefit from sale of subsidiary	(54.10%)
Other, net	(32.00%)	(0.40%)	(0.40%)
Effective tax rate	(173.90%)	28.90%	25.80%