Derivative Instruments - Gross amounts of derivative fair values contained in the consolidated balance sheet (Details) - USD ($) $ in Thousands		Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross amounts recognized in balance sheet		$ 23,244	$ 107,496
Gross amounts recognized in balance sheet		77,788	64,704
Collateral and counterparty netting, assets		(3,681)	(17,947)
Collateral and counterparty nettings, liabilities		(3,681)	(17,947)
Asset derivative fair value		19,563	89,549
Liability derivative fair value		74,107	46,757
Gross amounts not offset in the balance sheet - cash collateral	[1]	0	(36,980)
Derivative, Collateral, Right to Reclaim Cash	[1]	(25,188)	(5,810)
Derivative Asset		19,563	52,569
Derivative Liability		48,919	40,947
Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		207,028	1,396,801
Gross amounts recognized in balance sheet		3,834	85,175
Liability derivative notional amount		885,957	583,722
Gross amounts recognized in balance sheet		30,460	25,750
Derivatives designated as hedging instruments | Foreign currency exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		207,028	1,396,801
Gross amounts recognized in balance sheet		3,834	85,175
Liability derivative notional amount		885,957	583,722
Gross amounts recognized in balance sheet		30,460	25,750
Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		373,316	342,182
Gross amounts recognized in balance sheet		19,410	22,321
Liability derivative notional amount		5,216,710	4,472,756
Gross amounts recognized in balance sheet		47,328	38,954
Derivatives not designated as hedging instruments | Investment related derivatives | Interest rate exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		0	0
Gross amounts recognized in balance sheet		0	0
Liability derivative notional amount		131,674	0
Gross amounts recognized in balance sheet		34	0
Derivatives not designated as hedging instruments | Investment related derivatives | Foreign currency exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		64,307	10,049
Gross amounts recognized in balance sheet		1,170	190
Liability derivative notional amount		316,806	273,767
Gross amounts recognized in balance sheet		3,716	12,137
Derivatives not designated as hedging instruments | Investment related derivatives | Credit exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		5,000	32,500
Gross amounts recognized in balance sheet		231	1,077
Liability derivative notional amount		45,000	82,500
Gross amounts recognized in balance sheet		6,988	6,978
Derivatives not designated as hedging instruments | Investment related derivatives | Financial market exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		0	5
Gross amounts recognized in balance sheet		0	5
Liability derivative notional amount		4,607	0
Gross amounts recognized in balance sheet		79	0
Derivatives not designated as hedging instruments | Other non-investment derivatives | Foreign currency exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		181,300	181,300
Gross amounts recognized in balance sheet		393	1,208
Liability derivative notional amount		0	0
Gross amounts recognized in balance sheet		0	0
Derivatives not designated as hedging instruments | Other non-investment derivatives | Other credit derivatives
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		26,159	0
Gross amounts recognized in balance sheet		65	0
Liability derivative notional amount		0	24,490
Gross amounts recognized in balance sheet		0	340
Derivatives not designated as hedging instruments | Other non-investment derivatives | Guaranteed minimum income benefit contract
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		38,746	43,553
Gross amounts recognized in balance sheet		17,470	19,499
Liability derivative notional amount		38,746	43,553
Gross amounts recognized in balance sheet		17,470	19,499
Derivatives not designated as hedging instruments | Other non-investment derivatives | Modified coinsurance and funds withheld contracts
Derivatives, Fair Value [Line Items]
Asset derivative notional amount	[2]	52,804	59,775
Gross amounts recognized in balance sheet	[2]	0	0
Liability derivative notional amount	[2]	4,679,877	4,048,446
Gross amounts recognized in balance sheet	[2]	19,041	0
Derivatives not designated as hedging instruments | Other non-investment derivatives | Other
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		5,000	15,000
Gross amounts recognized in balance sheet		81	342
Liability derivative notional amount		0	0
Gross amounts recognized in balance sheet		$ 0	$ 0

[1] At September 30, 2017, the Company held cash collateral related to foreign currency derivative positions and certain other derivative positions of nil for derivatives in an asset position and paid cash collateral of $25.2 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 Unaudited 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 funds withheld contracts are included within the associated asset or liability at each period end on the face of the Unaudited 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 at September 30, 2017 is the cumulative net realized and unrealized loss on the life retrocession embedded derivative of $1.0 billion