Securities Sold Under Agreements to Repurchase (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Securities Sold Under Agreements to Repurchase
Maximum maturity period of securities sold under agreements to repurchase	1 year
Balance	$ 130,809	$ 139,024
Weighted average interest rate at end of period (as a percent)	0.13%	0.15%
Maximum outstanding at any month end	133,362	146,710
Average daily balance	$ 130,093	$ 132,150
Weighted average interest rate during period (as a percent)	0.14%	0.21%