Derivative Instruments - Gross amounts of derivative fair values contained in the consolidated balance sheet (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016
Derivatives Fair Value [Line Items]
Asset derivative fair value		$ 32,794	$ 107,496
Liability derivative fair value		62,896	64,704
Derivative assets - gross amounts offset in the balance sheet		(3,579)	(17,947)
Derivative liabilities - gross amounts offset in the balance sheet		(3,579)	(17,947)
Derivative assets - net amounts in the balance sheet		29,215	89,549
Derivative liabilities - net amounts in the balance sheet		59,317	46,757
Derivative assets - cash collateral not offset in the balance sheet	[1]	(3,920)	(36,980)
Derivative liabilities - cash collateral not offset in the balance sheet	[1]	(1,312)	(5,810)
Derivative assets - net amount		25,295	52,569
Derivative liabilities - net amount		58,005	40,947
Designated as Hedging Instrument
Derivatives Fair Value [Line Items]
Asset derivative notional amount		580,194	1,396,801
Asset derivative fair value		5,655	85,175
Liability derivative notional amount		339,221	583,722
Liability derivative fair value		3,968	25,750
Designated as Hedging Instrument | Foreign currency exposure
Derivatives Fair Value [Line Items]
Asset derivative notional amount		580,194	1,396,801
Asset derivative fair value		5,655	85,175
Liability derivative notional amount		339,221	583,722
Liability derivative fair value		3,968	25,750
Not Designated as Hedging Instrument
Derivatives Fair Value [Line Items]
Asset derivative notional amount		584,522	342,182
Asset derivative fair value		27,139	22,321
Liability derivative notional amount		5,269,279	4,472,756
Liability derivative fair value		58,928	38,954
Not Designated as Hedging Instrument | Investment Related Derivatives | Interest rate exposure
Derivatives Fair Value [Line Items]
Asset derivative notional amount		181,975	0
Asset derivative fair value		423	0
Liability derivative notional amount		0	0
Liability derivative fair value		0	0
Not Designated as Hedging Instrument | Investment Related Derivatives | Foreign currency exposure
Derivatives Fair Value [Line Items]
Asset derivative notional amount		263,722	10,049
Asset derivative fair value		6,931	190
Liability derivative notional amount		141,300	273,767
Liability derivative fair value		2,256	12,137
Not Designated as Hedging Instrument | Investment Related Derivatives | Credit exposure
Derivatives Fair Value [Line Items]
Asset derivative notional amount		5,000	32,500
Asset derivative fair value		187	1,077
Liability derivative notional amount		45,000	82,500
Liability derivative fair value		6,784	6,978
Not Designated as Hedging Instrument | Investment Related Derivatives | Financial market exposure
Derivatives Fair Value [Line Items]
Asset derivative notional amount		30,001	5
Asset derivative fair value		1,271	5
Liability derivative notional amount		6,998	0
Liability derivative fair value		42	0
Not Designated as Hedging Instrument | Other Non Investment Derivatives | Foreign currency exposure
Derivatives Fair Value [Line Items]
Asset derivative notional amount		0	181,300
Asset derivative fair value		0	1,208
Liability derivative notional amount		0	0
Liability derivative fair value		0	0
Not Designated as Hedging Instrument | Other Non Investment Derivatives | Credit exposure
Derivatives Fair Value [Line Items]
Asset derivative notional amount		0	0
Asset derivative fair value		0	0
Liability derivative notional amount		25,526	24,490
Liability derivative fair value		169	340
Not Designated as Hedging Instrument | Other Non Investment Derivatives | Guaranteed minimum income benefit contract
Derivatives Fair Value [Line Items]
Asset derivative notional amount		36,171	43,553
Asset derivative fair value		18,136	19,499
Liability derivative notional amount		36,171	43,553
Liability derivative fair value		18,136	19,499
Not Designated as Hedging Instrument | Other Non Investment Derivatives | Modified coinsurance and funds withheld contracts, including life retrocession embedded derivative
Derivatives Fair Value [Line Items]
Asset derivative notional amount	[2]	51,653	59,775
Asset derivative fair value	[2]	0	0
Liability derivative notional amount	[2]	5,014,284	4,048,446
Liability derivative fair value	[2]	31,541	0
Not Designated as Hedging Instrument | Other Non Investment Derivatives | Other
Derivatives Fair Value [Line Items]
Asset derivative notional amount		16,000	15,000
Asset derivative fair value		191	342
Liability derivative notional amount		0	0
Liability derivative fair value		$ 0	$ 0

[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.

[2] The fair value movements in derivative assets and liabilities relating to modified coinsurance and funds withheld contracts are included within the associated asset or liability at each period end on the face of the Consolidated Balance Sheets. Notional amounts associated with reinsurance agreements under which the Company assumes reinsurance risk are recorded as asset derivative notional amounts. Notional amounts associated with the GreyCastle Life Retro Arrangements and other outward reinsurance contracts under which the Company cedes risk are recorded as liability derivative notional amounts. Included in the liability derivative notional amount as of December 31, 2017 is the cumulative net realized and unrealized loss on the life retrocession embedded derivative of $1.1 billion.