Federal Funds Purchased and Securities Sold Under Agreements to Repurchase (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Amount
At period-end	$ 238,621	$ 180,436
Federal funds purchased and securities sold under repurchase agreements
Amount
At period-end	238,621	180,436	191,017
Average for the year	229,185	210,098	214,096
Maximum month-end balance	$ 254,104	$ 243,960	$ 266,818
Rate
At period-end (as a percent)	0.17%	0.25%	0.28%
Average for the year (as a percent)	0.20%	0.25%	0.29%
Federal funds purchased and securities sold under repurchase agreements | Minimum
Information concerning federal funds purchased and securities sold under repurchase agreements
Maturity period from the transaction date	1 day
Federal funds purchased and securities sold under repurchase agreements | Maximum
Information concerning federal funds purchased and securities sold under repurchase agreements
Maturity period from the transaction date	3 days
Maturity period as per policies	9 months