Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Fair Value Measurements
Transfers between levels of fair value hierarchy	$ 0
Financial assets and financial liabilities measured at fair value
Available for sale	907,422	1,001,497
Impaired loans	37,405	55,593
OREO	2,156	3,450
U.S. Treasury securities
Financial assets and financial liabilities measured at fair value
Available for sale	103,338	104,656
Obligations of U.S. government corporations and agencies
Financial assets and financial liabilities measured at fair value
Available for sale	288,369	370,194
Obligations of states and political subdivisions
Financial assets and financial liabilities measured at fair value
Available for sale	293,287	280,288
Residential mortgage-backed securities
Financial assets and financial liabilities measured at fair value
Available for sale	190,133	217,715
Corporate debt securities
Financial assets and financial liabilities measured at fair value
Available for sale	25,490	24,714
Mutual funds and other equity securities
Financial assets and financial liabilities measured at fair value
Available for sale	6,805	3,930
Recurring basis | Level 1 Inputs
Financial assets and financial liabilities measured at fair value
Available for sale	6,805	28,644
Recurring basis | Level 1 Inputs | Corporate debt securities
Financial assets and financial liabilities measured at fair value
Available for sale		24,714
Recurring basis | Level 1 Inputs | Mutual funds and other equity securities
Financial assets and financial liabilities measured at fair value
Available for sale	6,805	3,930
Recurring basis | Level 2 Inputs
Financial assets and financial liabilities measured at fair value
Available for sale	900,617	972,853
Recurring basis | Level 2 Inputs | U.S. Treasury securities
Financial assets and financial liabilities measured at fair value
Available for sale	103,338	104,656
Recurring basis | Level 2 Inputs | Obligations of U.S. government corporations and agencies
Financial assets and financial liabilities measured at fair value
Available for sale	288,369	370,194
Recurring basis | Level 2 Inputs | Obligations of states and political subdivisions
Financial assets and financial liabilities measured at fair value
Available for sale	293,287	280,288
Recurring basis | Level 2 Inputs | Residential mortgage-backed securities
Financial assets and financial liabilities measured at fair value
Available for sale	190,133	217,715
Recurring basis | Level 2 Inputs | Corporate debt securities
Financial assets and financial liabilities measured at fair value
Available for sale	25,490
Recurring basis | Total Fair Value
Financial assets and financial liabilities measured at fair value
Available for sale	907,422	1,001,497
Recurring basis | Total Fair Value | U.S. Treasury securities
Financial assets and financial liabilities measured at fair value
Available for sale	103,338	104,656
Recurring basis | Total Fair Value | Obligations of U.S. government corporations and agencies
Financial assets and financial liabilities measured at fair value
Available for sale	288,369	370,194
Recurring basis | Total Fair Value | Obligations of states and political subdivisions
Financial assets and financial liabilities measured at fair value
Available for sale	293,287	280,288
Recurring basis | Total Fair Value | Residential mortgage-backed securities
Financial assets and financial liabilities measured at fair value
Available for sale	190,133	217,715
Recurring basis | Total Fair Value | Corporate debt securities
Financial assets and financial liabilities measured at fair value
Available for sale	25,490	24,714
Recurring basis | Total Fair Value | Mutual funds and other equity securities
Financial assets and financial liabilities measured at fair value
Available for sale	6,805	3,930
Non-recurring basis | Level 3 Inputs
Financial assets and financial liabilities measured at fair value
Impaired loans	3,963	2,284
OREO	1,192	511
Non-recurring basis | Total Fair Value
Financial assets and financial liabilities measured at fair value
Impaired loans	3,963	2,284
OREO	$ 1,192	$ 511