Derivative Instruments - Narrative (Details) $ in Thousands		Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2012 Contracts
Derivative [Line Items]
Gross amounts not offset in the balance sheet - cash collateral	[1]	$ 43,420	$ 3,920
Derivative, Collateral, Right to Reclaim Cash	[1]	3,089	$ 1,312
Number of Derivative Contracts Settled | Contracts				5
Life Funds Withheld Assets
Derivative [Line Items]
Cumulative net realized and unrealized loss on life retrocession embedded derivative		$ (900,000)

[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.