Difference between Federal Income Tax Rate and Effective Income Tax Rate (Detail)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reconciliation of Statutory Federal Tax Rate [Line Items]
Federal income tax rate	35.00%	35.00%	35.00%
Foreign tax credits	(0.60%)	(0.40%)
Income subject to dividends-received deduction	(1.60%)	(1.60%)	(1.30%)
Tax-exempt interest	(6.50%)	(6.80%)	(6.50%)
State taxes, net of federal tax benefit	0.60%	0.20%	0.40%
Prior period adjustment	2.40%	(0.20%)	0.10%
Other, net	(0.40%)	(0.40%)	(0.70%)
Effective tax rate	28.90%	25.80%	27.00%