Income Taxes (Details 1) (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
Expected income tax expense									$ 10,917	$ 11,027	$ 9,777
(Reductions) increases resulting from:
Tax-exempt income									(3,633)	(3,760)	(3,273)
State taxes, net of federal benefit									493	742	657
Other, net									110	488	462
Total income tax expense	$ 1,607	$ 1,890	$ 2,205	$ 2,185	$ 1,683	$ 2,358	$ 2,229	$ 2,227	$ 7,887	$ 8,497	$ 7,623