Securities Sold Under Agreements to Repurchase (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Securities Sold Under Agreements to Repurchase
Maximum maturity period of securities sold under agreements to repurchase	1 year
Balance	$ 139,024	$ 127,867
Weighted average interest rate at end of period (as a percent)	0.15%	0.21%
Maximum outstanding at any month end	146,710	142,557
Average daily balance	$ 132,150	$ 127,095
Weighted average interest rate during period (as a percent)	0.21%	0.29%