Derivatives - Assets And Collateral Received (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Gross amounts of recognized assets	$ 54,476	$ 81,634
Gross amounts not offset in the Statements of Condition
Derivative assets not subject to master netting agreements	20,100	10,200
Derivatives, interest rate contracts
Derivative [Line Items]
Gross amounts of recognized assets	34,414	71,458
Gross amounts offset in the Statements of Condition	0	0
Net amounts of assets presented in the Statements of Condition	34,414	71,458
Gross amounts not offset in the Statements of Condition
Derivative liabilities available for offset	(8,590)	(17,278)
Collateral received	(25,156)	(51,271)
Net amount	$ 668	$ 2,909