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, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed expected tax expense									$ 11,457	$ 34,590	$ 30,397
Tax-exempt interest									(4,437)	(5,992)	(8,295)
Dividends received deduction									(1,065)	(718)	(598)
Proration									825	1,006	1,334
Current state and local income taxes, net of federal income tax									221	63	93
Other									136	302	759
Income tax expense (benefit)	$ 2,122	$ 4,476	$ 5,032	$ (4,493)	$ 7,592	$ 7,091	$ 8,223	$ 6,345	$ 7,137	$ 29,251	$ 23,690
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed expected tax expense									35.00%	35.00%	35.00%
Tax-exempt interest									(13.60%)	(6.10%)	(9.60%)
Dividends received deduction									(3.30%)	(0.70%)	(0.70%)
Proration									2.50%	1.00%	1.50%
Current state and local income taxes, net of federal income tax									0.70%	0.10%	0.10%
Other									0.40%	0.30%	0.90%
Actual tax expense and rate									21.80%	29.60%	27.30%