Federal Income Taxes - Effective Income Tax Reconciliation (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income tax expense before tax on equity in earnings of unconsolidated affiliates									$ 145,819	$ 29,643	$ 114,921
Tax on equity in earnings of unconsolidated affiliates									21,735	4,469	30,336
Total expected income tax expense at the statutory rate									167,554	34,112	145,257
Tax-exempt investment income									(3,969)	(3,323)	(6,887)
Deferred tax change									(519)	(4,354)	(217,622)
Dividend exclusion									(3,628)	(4,080)	(8,701)
Miscellaneous tax credits, net									(7,090)	(7,802)	(9,524)
Low income housing tax credit expense									6,394	6,231	5,263
Change in valuation allowance									383	2,700	0
Tax accrual adjustment									5,350	(2,893)	0
Return to provision									1,007	(20,301)	0
Other items, net									(65)	1,155	1,905
Provision for federal income tax before interest expense									165,417	1,445	(90,309)
Interest benefit									0	0	(2,686)
Total provision (benefit) for federal income taxes	$ 53,617	$ 22,475	$ 21,948	$ 67,377	$ (40,920)	$ 19,219	$ 21,957	$ 1,189	$ 165,417	$ 1,445	$ (92,995)
Income tax expense before tax on equity in earnings of unconsolidated affiliates, percentage									18.30%	18.20%	27.70%
Tax on equity in earnings of unconsolidated affiliates, percentage									2.70%	2.80%	7.30%
Total expected income tax expense at the statutory rate, percentage									21.00%	21.00%	35.00%
Tax-exempt investment income, percentage									(0.50%)	(2.00%)	(1.70%)
Deferred tax change, percentage									(0.10%)	(2.70%)	(52.40%)
Dividend exclusion, percentage									(0.50%)	(2.50%)	(2.10%)
Miscellaneous tax credits, net, percentage									(0.90%)	(4.80%)	(2.30%)
Low income housing tax credit expense, percentage									0.80%	3.80%	1.30%
Change in valuation allowance, percentage									0.00%	1.70%	0.00%
Tax accrual adjustment, percentage									0.70%	(1.80%)	0.00%
Return to provision, percentage									0.10%	(12.50%)	0.00%
Other items, net, percentage									(0.10%)	0.60%	0.50%
Provision for federal income tax before interest expense, percentage									20.50%	0.80%	(21.70%)
Interest benefit, percentage									0.00%	0.00%	(0.60%)
Total provision (benefit) for federal income taxes, percentage									20.50%	0.80%	(22.30%)