Derivative Instruments - Summary of contingent credit features (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Derivative [Line Items]
Collateral posted to counterparty	[1]	$ 3,089	$ 1,312
Contracts with downgrade provisions [Member]
Derivative [Line Items]
Aggregate fair value of derivative agreements with downgrade provisions in a net liability position		0	7,464
Collateral posted to counterparty		$ 0	$ 40

[1] At June 30, 2018, the Company held cash collateral related to foreign currency derivative positions and certain other derivative positions of $43.4 million for derivatives in an asset position and paid cash collateral of $3.1 million for derivatives in a liability position. At December 31, 2017, the Company held cash collateral related to a foreign currency derivative position 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. The assets and liabilities related to the net collateral paid or held were recorded as Other assets and Other liabilities within the Unaudited Consolidated Balance Sheets as the collateral and derivative positions are not intended to be settled on a net basis.