Reconciliation of Total Income Taxes Applicable to Pre-Tax Operating Income and Amounts Computed by Applying Federal Statutory Income Tax Rate to Pre-Tax Operating Income (Detail) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed Expected Tax Expense									$ 10,201	$ 26,182	$ 41,128
Tax-Exempt Interest									(3,356)	(5,394)	(4,559)
Dividends Received Deduction									(2,041)	(4,689)	(3,812)
Proration of DRD and Tax-Exempt Interest									1,330	1,512	1,256
State and Local Income Taxes, Net of Federal Income Tax Deduction									738	497	415
Stock Compensation Fair Market Value									(1,711)	(5,027)
Change in Valuation Allowance									(774)	1,182
Deferred Tax Expense due to Tax Law Changes									(506)	17,107
Transition Tax									(1,314)	2,587
Uncertain Tax Position - FTC IRS Audit									7,029
Non-Deductible Expenses									1,197	405	406
Return to Provision									2,326	(158)	1
Other									1,217	108	(52)
Actual Tax Expense and Rate	$ 879	$ (462)	$ 7,684	$ 6,235	$ 36,555	$ (16,864)	$ 6,971	$ 7,650	$ 14,336	$ 34,312	$ 34,783
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed Expected Tax Expense									21.00%	35.00%	35.00%
Tax-Exempt Interest									(6.90%)	(7.20%)	(3.90%)
Dividends Received Deduction									(4.20%)	(6.30%)	(3.20%)
Proration of DRD and Tax-Exempt Interest									2.70%	2.00%	1.10%
State and Local Income Taxes, Net of Federal Income Tax Deduction									1.50%	0.70%	0.40%
Stock Compensation Fair Market Value									(3.50%)	(6.70%)
Change in Valuation Allowance									(1.60%)	1.60%
Deferred Tax Expense due to Tax Law Changes									(1.00%)	22.90%
Transition Tax									(2.70%)	3.50%
Uncertain Tax Position - FTC IRS Audit									14.50%
Non-Deductible Expenses									2.50%	0.50%	0.20%
Return to Provision									4.80%	(0.20%)	0.00%
Other									2.40%	0.10%	0.00%
Actual Tax Expense and Rate									29.50%	45.90%	29.60%