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, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed expected tax expense									$ 32,299	$ 32,109	$ 11,457
Tax-exempt interest									(3,839)	(4,443)	(4,437)
Dividends received deduction									(897)	(799)	(1,065)
Proration									710	786	825
Current state and local income taxes, net of federal income tax									290	95	221
Other									244	226	136
Income tax expense (benefit)	$ 6,076	$ 9,804	$ 6,284	$ 6,643	$ 13,243	$ 5,225	$ 6,225	$ 3,281	$ 28,807	$ 27,974	$ 7,137
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed expected tax expense									35.00%	35.00%	35.00%
Tax-exempt interest									(4.20%)	(4.80%)	(13.60%)
Dividends received deduction									(1.00%)	(0.90%)	(3.30%)
Proration									0.80%	0.90%	2.50%
Current state and local income taxes, net of federal income tax									0.30%	0.10%	0.70%
Other									0.30%	0.20%	0.50%
Actual tax expense and rate									31.20%	30.50%	21.80%