Derivative Instruments - Schedule of settlement of fair value hedges (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative liabilities - cash collateral not offset in the balance sheet	[1]	$ 1,312	$ 5,810
Deposit Liabilities | Fair Value Hedging Member
Derivative [Line Items]
Fair Value Hedge Cumulative Reduction To Expense		122,118	113,292
Fair Value Hedge Remaining Balance		$ 111,077	$ 119,903
Derivative, Average Remaining Maturity		18 years 8 months 12 days	20 years 1 month 6 days

[1] At December 31, 2017, the Company held cash collateral related to foreign currency derivative positions and certain other derivative positions of $3.9 million for derivatives in an asset position and paid cash collateral of $1.3 million for derivatives in a liability position. At December 31, 2016, the Company held cash collateral related to a foreign currency derivative position and certain other derivative positions of $37.0 million for derivatives in an asset position and paid cash collateral of $5.8 million for derivatives in a liability position. The assets and liabilities related to the net collateral paid or held were recorded as Other assets and Other liabilities within the Consolidated Balance Sheets as the collateral and derivative positions are not intended to be settled on a net basis.