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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed Expected Tax Expense									$ 26,182	$ 41,128	$ 41,116
Tax-Exempt Interest									(5,394)	(4,559)	(4,987)
Dividends Received Deduction									(4,689)	(3,812)	(2,086)
Proration of DRD and Tax-Exempt Interest									1,512	1,256	1,061
Current State and Local Income Taxes, Net of Federal Income Tax Deduction									497	415	959
Stock Compensation Fair Market Value									(5,027)
Change in Valuation Allowance									1,182
Deferred Tax Expense due to Tax Law Changes									17,107
Transition Tax									2,587
Other									355	355	354
Actual Tax Expense and Rate	$ 36,555	$ (16,864)	$ 6,971	$ 7,650	$ 9,866	$ 7,875	$ 7,053	$ 9,989	$ 34,312	$ 34,783	$ 36,417
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed Expected Tax Expense									35.00%	35.00%	35.00%
Tax-Exempt Interest									(7.20%)	(3.90%)	(4.20%)
Dividends Received Deduction									(6.30%)	(3.20%)	(1.80%)
Proration of DRD and Tax-Exempt Interest									2.00%	1.10%	0.90%
Current State and Local Income Taxes, Net of Federal Income Tax Deduction									0.70%	0.40%	0.80%
Stock Compensation Fair Market Value									(6.70%)
Change in Valuation Allowance									1.60%
Deferred Tax Expense due to Tax Law Changes									22.90%
Transition Tax									3.50%
Other									0.40%	0.20%	0.30%
Actual Tax Expense and Rate									45.90%	29.60%	31.00%