Derivatives - Liabilities and Collateral Pledged (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Gross amounts of recognized liabilities	$ 170,324	$ 85,061
Gross amounts not offset in the Statements of Condition
Derivative liabilities not subject to master netting agreements	53,100	15,200
Derivatives, interest rate contracts
Derivative [Line Items]
Gross amounts of recognized liabilities	117,241	69,842
Gross amounts offset in the Statements of Condition	0	0
Net amounts of liabilities presented in the Statements of Condition	117,241	69,842
Gross amounts not offset in the Statements of Condition
Derivative assets available for offset	(8,590)	(17,278)
Collateral pledged	(65,354)	(51,801)
Net amount	$ 43,297	$ 763