Federal Funds Purchased and Securities Sold Under Agreements to Repurchase (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Amount
At period-end	$ 286,857	$ 313,773
Rate
Repurchase agreement	211,100	238,300
Carrying amount of the securities pledged to collateralize repurchase agreements	211,100	238,300
Federal funds purchased and securities sold under repurchase agreements
Amount
At period-end	286,857	313,773	$ 288,231
Average for the year	325,713	320,901	291,428
Maximum month-end balance	$ 401,786	$ 334,260	$ 320,373
Rate
At period-end (as a percent)	0.45%	0.24%	0.19%
Average for the year (as a percent)	0.33%	0.18%	0.14%
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