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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Income Tax Expense (Benefit), Continuing Operations, Income Tax Reconciliation
Computed expected tax expense									$ 32,109	$ 11,457	$ 34,590
Tax-exempt interest									(4,443)	(4,437)	(5,992)
Dividends received deduction									(799)	(1,065)	(718)
Proration									786	825	1,006
Current state and local income taxes, net of federal income tax									95	221	63
Other									226	136	302
Income tax expense (benefit)	$ 13,243	$ 5,225	$ 6,225	$ 3,281	$ 2,122	$ 4,476	$ 5,032	$ (4,493)	$ 27,974	$ 7,137	$ 29,251
Effective Income Tax Rate, Continuing Operations, Tax Rate Reconciliation
Computed expected tax expense									35.00%	35.00%	35.00%
Tax-exempt interest									(4.80%)	(13.60%)	(6.10%)
Dividends received deduction									(0.90%)	(3.30%)	(0.70%)
Proration									0.90%	2.50%	1.00%
Current state and local income taxes, net of federal income tax									0.10%	0.70%	0.10%
Other									0.20%	0.50%	0.30%
Actual tax expense and rate									30.50%	21.80%	29.60%