Securities Sold Under Agreements to Repurchase (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Securities Sold Under Agreements to Repurchase
Maximum maturity period of securities sold under agreements to repurchase	1 year
Balance	$ 176,961	$ 198,893
Weighted average interest rate at end of period (as a percent)	0.11%	0.14%
Maximum outstanding at any month end	$ 191,531	$ 198,893
Average daily balance	$ 177,937	$ 148,452
Weighted average interest rate during period (as a percent)	0.10%	0.12%