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, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed expected tax expense									$ 41,116	$ 49,187	$ 32,299
Tax-exempt interest									(4,987)	(4,771)	(3,839)
Dividends received deduction									(2,086)	(1,257)	(897)
Proration of DRD and Tax-exempt interest									1,061	904	710
Current state and local income taxes, net of federal income tax deduction									959	674	290
Other									354	470	244
Actual tax expense and rate	$ 6,072	$ 8,723	$ 9,195	$ 12,427	$ 8,823	$ 15,032	$ 7,998	$ 13,354	$ 36,417	$ 45,207	$ 28,807
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed expected tax expense									35.00%	35.00%	35.00%
Tax-exempt interest									(4.20%)	(3.40%)	(4.20%)
Dividends received deduction									(1.80%)	(0.90%)	(1.00%)
Proration of DRD and Tax-exempt interest									0.90%	0.60%	0.80%
Current state and local income taxes, net of federal income tax deduction									0.80%	0.50%	0.30%
Other									0.30%	0.40%	0.30%
Actual tax expense and rate									31.00%	32.20%	31.20%