Derivative Instruments - Narrative (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014		Jun. 30, 2014
Derivative [Line Items]
Underwriting profit (loss)		$ 547,325		$ 587,409		$ 585,907
Other Comprehensive Income (Loss), Net of Tax		28,930		(797,842)		747,801
Derivative assets - cash collateral not offset in the balance sheet	[1]	36,980		30,958
Collateral posted to counterparty	[1]	5,810		0
Net results from structured products		14,280	[2]	17,991	[3]	54,209	[4]
Asset derivative fair value		107,496		89,314
Net realized and unrealized gains (losses) on derivative instruments		2,521		53,123		29,886
Fair Value Hedging Member | Deposit Liabilities
Derivative [Line Items]
Fair Value Hedge Remaining Balance		119,903		129,454
Fair Value Hedging Member | Structured Indemnity | Deposit Liabilities
Derivative [Line Items]
Fair Value Hedge Remaining Balance						0		$ 47,000
Life Funds Withheld Assets
Derivative [Line Items]
Cumulative net realized and unrealized gains (losses) on life retrocession embedded derivative		(937,700)
Net realized and unrealized gains (losses) on derivative instruments		(540,090)		(151,691)		(488,222)
Not Designated as Hedging Instrument
Derivative [Line Items]
Asset derivative notional amount		342,182		441,843
Asset derivative fair value	[5]	22,321		25,025
Other Non Investment Derivatives | Not Designated as Hedging Instrument | Other Credit Derivatives [Member]
Derivative [Line Items]
Asset derivative notional amount		0		29,874
Asset derivative fair value	[5]	0		31
Interest expense | Structured Indemnity
Derivative [Line Items]
Net results from structured products						28,700
Interest expense | Property and casualty operations
Derivative [Line Items]
Net results from structured products		(39,700)		(42,200)		(12,900)
XL Re Life (SAC) Ltd [Member]
Derivative [Line Items]
Other Comprehensive Income (Loss), Net of Tax		0		0
XL Re Life (SAC) Ltd [Member] | Life Funds Withheld Assets
Derivative [Line Items]
Underwriting profit (loss)		0		605		11,649
Other Comprehensive Income (Loss), Net of Tax		0		605		11,649
Net realized and unrealized gains (losses) on derivative instruments		$ (540,090)		$ (151,691)		$ (488,222)

[1] As of December 31, 2016, the Company held cash collateral related to foreign currency derivative positions 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. As of December 31, 2015, the Company held cash collateral related to a foreign currency derivative position and certain other derivative positions of $31.0 million for derivatives in an asset position and paid cash collateral of nil 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 net results from P&C structured products include net investment income and interest expense of $54.0 million and $39.7 million, respectively.

[3] The net results from P&C structured products include net investment income and interest expense of $60.4 million and $42.2 million, respectively.

[4] The net results from P&C structured products include net investment income and interest expense of $68.0 million and $12.9 million, respectively.

[5] Derivative instruments in an asset or liability position are included within Other assets or Other liabilities, respectively, in the balance sheets on a net basis where the Company has both a legal right of offset and the intention to settle the contracts on a net basis. The Company often enters into different types of derivative contracts with a single counterparty and these contracts are covered under netting agreements.