Federal Income Taxes - Effective Income Tax Reconciliation (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Income Tax Disclosure [Abstract]
Income tax expense before tax on equity in earnings of unconsolidated affiliates	$ 21,819	$ 21,267	$ 81,606	$ 25,632
Tax on equity in earnings of unconsolidated affiliates	3,526	1,348	12,023	1,234
Total expected income tax expense at the statutory rate	25,345	22,615	93,629	26,866
Tax-exempt investment income	(1,109)	(836)	(1,918)	(1,679)
Deferred tax change	0	(600)	0	(909)
Dividend exclusion	(860)	(1,001)	(1,781)	(1,986)
Miscellaneous tax credits, net	(3,299)	(2,529)	(4,991)	(4,742)
Low income housing tax credit expense	1,783	1,252	2,941	2,504
Change in valuation allowance	165	2,700	165	2,700
Other items, net	(77)	356	1,280	392
Provision for federal income taxes	$ 21,948	$ 21,957	$ 89,325	$ 23,146
Income tax expense before tax on equity in earnings of unconsolidated affiliates, percentage	18.10%	20.00%	18.30%	20.00%
Tax on equity in earnings of unconsolidated affiliates, percentage	2.90%	1.00%	2.70%	1.00%
Total expected income tax expense at the statutory rate, percentage	21.00%	21.00%	21.00%	21.00%
Tax-exempt investment income, percentage	(0.90%)	(0.80%)	(0.40%)	(1.30%)
Deferred tax change, percentage	0.00%	(0.60%)	0.00%	(0.70%)
Dividend exclusion, percentage	(0.70%)	(0.90%)	(0.40%)	(1.60%)
Miscellaneous tax credits, net, percentage	(2.70%)	(2.30%)	(1.10%)	(3.70%)
Low income housing tax credit expense, percentage	1.50%	1.20%	0.60%	2.00%
Change in valuation allowance, percentage	0.10%	2.50%	0.00%	2.10%
Other items, net, percentage	(0.10%)	0.30%	0.30%	0.30%
Provision for federal income tax before interest expense, percentage	18.20%	20.40%	20.00%	18.10%