Income Tax (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
Currently Payable:
Federal									$ (23)	$ 399	$ (1,780)
Deferred:
Federal									15,890	8,256	(1,810)
Total Income Tax Expense	3,153	3,926	3,288	5,500	2,299	2,561	1,396	2,399	15,867	8,655	(3,590)
Reconciliation of Federal Statutory to Actual Tax Expense:
Federal statutory income tax at 35%									21,347	11,867	1,162
Tax-exempt interest income									(3,716)	(3,714)	(3,733)
Non-deductible interest expense										649	421
Stock compensation									76	69	150
Earnings on life insurance									(1,187)	(899)	(725)
Tax credits									(73)	(99)	(116)
Other									(580)	782	(749)
Total Income Tax Expense	3,153	3,926	3,288	5,500	2,299	2,561	1,396	2,399	15,867	8,655	(3,590)
Tax expense applicable to security gains and losses, including unrealized losses relating to other-than-temporary charges									$ 836	$ 714	$ 652