Derivative Instruments - Narrative (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Derivative [Line Items]
Gross amounts not offset in the balance sheet - cash collateral	[1]	$ 5,520		$ 30,958
Derivative, Collateral, Right to Reclaim Cash	[1]	4,230		0
Derivative liability, Gross amounts recognized in balance sheet	[1]	88,688		45,495
Derivative asset, Gross amounts recognized in balance sheet	[1]	76,604		89,314
Net realized and unrealized gains (losses) on derivative instruments		(3,622)	$ 16,521
Underwriting profit (loss)		172,037	139,207
Comprehensive income (loss)		407,941	220,712
Derivatives not designated as hedging instruments
Derivative [Line Items]
Liability derivative notional amount		4,782,138		4,909,732
Asset derivative notional amount		500,693		441,843
Derivative liability, Gross amounts recognized in balance sheet	[1]	34,301		33,554
Derivative asset, Gross amounts recognized in balance sheet	[1]	25,897		25,025
Other non-investment derivatives | Derivatives not designated as hedging instruments | Other credit derivatives
Derivative [Line Items]
Liability derivative notional amount		30,090		0
Asset derivative notional amount		0		29,874
Derivative liability, Gross amounts recognized in balance sheet	[1]	28		0
Derivative asset, Gross amounts recognized in balance sheet	[1]	0		$ 31
Life Funds Withheld Assets
Derivative [Line Items]
Embedded Derivative, Accumulated Net Gain (Loss) on Embedded Derivative		681,800
Net realized and unrealized gains (losses) on derivative instruments		(236,080)	(229,367)
XL Re Life (SAC) Ltd
Derivative [Line Items]
Comprehensive income (loss)		0
XL Re Life (SAC) Ltd | Life Funds Withheld Assets
Derivative [Line Items]
Net realized and unrealized gains (losses) on derivative instruments		(236,080)	(229,367)
Underwriting profit (loss)	[2]	0	603
Comprehensive income (loss)		$ 0	$ 603

[1] 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.

[2] The underwriting profit of $0.6 million relates to a premium adjustment during the three months ended March 31, 2015 relating to the GreyCastle Life Retro Arrangements transaction. Excluding this transaction, the impact to comprehensive income relating to the GreyCastle Life Retro Arrangements was nil for the three months ended March 31, 2015.