Derivative Instruments - Summary of contingent credit features (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Collateral posted to counterparty	[1]	$ 1,312	$ 5,810
Contracts with downgrade provisions [Member]
Derivative [Line Items]
Aggregate fair value of derivative agreements with downgrade provisions in a net liability position		7,464	14,130
Collateral posted to counterparty		$ 40	$ 4,630

[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.