Securities Sold Under Agreements to Repurchase (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Securities Sold Under Agreements to Repurchase
Maximum maturity period of securities sold under agreements to repurchase	1 year
Balance	$ 148,238	$ 139,024
Weighted average interest rate at end of period (as a percent)	0.11%	0.15%
Maximum outstanding at any month end	148,238	146,710
Average daily balance	$ 131,911	$ 132,150
Weighted average interest rate during period (as a percent)	0.13%	0.21%