Derivative Instruments - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Derivative [Line Items]
Derivative assets - cash collateral not offset in the balance sheet	[1]	$ 3,920	$ 36,980
Collateral posted to counterparty	[1]	1,312	5,810
Comprehensive income (loss)		173,885	28,930	$ (797,842)
Life Funds Withheld Assets
Derivative [Line Items]
Cumulative net realized and unrealized gains (losses) on life retrocession embedded derivative		(1,100,000)
GreyCastle Life Reinsurance SAC Ltd
Derivative [Line Items]
Comprehensive income (loss)		0	0
GreyCastle Life Reinsurance SAC Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Comprehensive income (loss)		$ 0	$ 0	$ 605

[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.