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, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed expected tax expense									$ 49,187	$ 32,299	$ 32,109
Tax-exempt interest									(4,771)	(3,839)	(4,443)
Dividends received deduction									(1,257)	(897)	(799)
Proration of DRD and Tax-exempt interest									904	710	786
Current state and local income taxes, net of federal income tax deduction									674	290	95
Other									470	244	226
Actual tax expense and rate	$ 8,823	$ 15,032	$ 7,998	$ 13,354	$ 6,076	$ 9,804	$ 6,284	$ 6,643	$ 45,207	$ 28,807	$ 27,974
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed expected tax expense									35.00%	35.00%	35.00%
Tax-exempt interest									(3.40%)	(4.20%)	(4.80%)
Dividends received deduction									(0.90%)	(1.00%)	(0.90%)
Proration of DRD and Tax-exempt interest									0.60%	0.80%	0.90%
Current state and local income taxes, net of federal income tax deduction									0.50%	0.30%	0.10%
Other									0.30%	0.30%	0.20%
Actual tax expense and rate									32.20%	31.20%	30.50%