Federal Income Taxes - Effective Income Tax Reconciliation (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Income tax on pre-tax income	$ 22,050	$ 33,171	$ 91,284	$ 82,308
Tax-exempt investment income	(1,863)	(1,742)	(5,688)	(4,897)
Dividend exclusion	(1,841)	(1,700)	(5,788)	(5,253)
Miscellaneous tax credits, net	(2,256)	(2,658)	(6,728)	(5,873)
Low income housing tax credit expense	1,307	1,786	3,792	3,432
Other items, net	737	(322)	2,162	(621)
Total provision for federal income taxes	$ 18,134	$ 28,535	$ 79,034	$ 69,096
Income tax on pre-tax income	35.00%	35.00%	35.00%	35.00%
Tax-exempt investment income	(3.00%)	(1.80%)	(2.20%)	(2.10%)
Dividend exclusion	(2.90%)	(1.80%)	(2.20%)	(2.20%)
Miscellaneous tax credits, net	(3.60%)	(2.80%)	(2.60%)	(2.50%)
Low income housing tax credit expense	2.10%	1.90%	1.50%	1.50%
Other items, net	1.20%	(0.30%)	0.80%	(0.30%)
Total, Rate	28.80%	30.20%	30.30%	29.40%