Derivative Instruments - Gross amounts of derivative fair values contained in the consolidated balance sheet (Details) - USD ($) $ in Thousands		Jun. 30, 2018	Dec. 31, 2017
Derivatives, Fair Value [Line Items]
Gross amounts recognized in balance sheet		$ 82,544	$ 32,794
Gross amounts recognized in balance sheet		47,651	62,896
Counterparty netting, assets		(7,627)	(3,579)
Counterparty netting, liabilities		(7,627)	(3,579)
Asset derivative fair value		74,917	29,215
Liability derivative fair value		40,024	59,317
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)
Derivative Asset		31,497	25,295
Derivative Liability		36,935	58,005
Derivatives designated as hedging instruments
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		953,438	580,194
Gross amounts recognized in balance sheet		60,325	5,655
Liability derivative notional amount		166,497	339,221
Gross amounts recognized in balance sheet		4,277	3,968
Derivatives designated as hedging instruments | Foreign currency exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		953,438	580,194
Gross amounts recognized in balance sheet		60,325	5,655
Liability derivative notional amount		166,497	339,221
Gross amounts recognized in balance sheet		4,277	3,968
Derivatives not designated as hedging instruments
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		268,123	584,522
Gross amounts recognized in balance sheet		22,219	27,139
Liability derivative notional amount		5,012,595	5,269,279
Gross amounts recognized in balance sheet		43,374	58,928
Derivatives not designated as hedging instruments | Investment related derivatives | Interest rate exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		0	181,975
Gross amounts recognized in balance sheet		0	423
Liability derivative notional amount		239,109	0
Gross amounts recognized in balance sheet		387	0
Derivatives not designated as hedging instruments | Investment related derivatives | Foreign currency exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		126,239	263,722
Gross amounts recognized in balance sheet		1,958	6,931
Liability derivative notional amount		211,809	141,300
Gross amounts recognized in balance sheet		6,360	2,256
Derivatives not designated as hedging instruments | Investment related derivatives | Credit exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		5,000	5,000
Gross amounts recognized in balance sheet		65	187
Liability derivative notional amount		45,000	45,000
Gross amounts recognized in balance sheet		5,730	6,784
Derivatives not designated as hedging instruments | Investment related derivatives | Financial market exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		34,580	30,001
Gross amounts recognized in balance sheet		2,317	1,271
Liability derivative notional amount		0	6,998
Gross amounts recognized in balance sheet		0	42
Derivatives not designated as hedging instruments | Other non-investment derivatives | Foreign currency exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		0	0
Gross amounts recognized in balance sheet		0	0
Liability derivative notional amount		24,534	0
Gross amounts recognized in balance sheet		468	0
Derivatives not designated as hedging instruments | Other non-investment derivatives | Credit exposure
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		19,553	0
Gross amounts recognized in balance sheet		57	0
Liability derivative notional amount		0	25,526
Gross amounts recognized in balance sheet		0	169
Derivatives not designated as hedging instruments | Other non-investment derivatives | Guaranteed minimum income benefit contract
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		35,400	36,171
Gross amounts recognized in balance sheet		17,822	18,136
Liability derivative notional amount		35,400	36,171
Gross amounts recognized in balance sheet		17,822	18,136
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]	47,351	51,653
Gross amounts recognized in balance sheet	[2]	0	0
Liability derivative notional amount	[2]	4,456,743	5,014,284
Gross amounts recognized in balance sheet	[2]	12,607	31,541
Derivatives not designated as hedging instruments | Other non-investment derivatives | Other
Derivatives, Fair Value [Line Items]
Asset derivative notional amount		0	16,000
Gross amounts recognized in balance sheet		0	191
Liability derivative notional amount		0	0
Gross amounts recognized in balance sheet		$ 0	$ 0

[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.

[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 in 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 June 30, 2018 is the cumulative net realized and unrealized loss on the life retrocession embedded derivative of $0.9 billion