Guarantor Financial Information - Cash Flow (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$ 33,319		$ 943,906		$ 644,481
Proceeds from sale of fixed maturities and short-term investments	15,702,126		12,757,580		16,868,228
Proceeds from redemption of fixed maturities and short-term investments	3,470,496		3,847,709		3,751,174
Proceeds from sale of equity securities	757,549		848,779		664,735
Purchases of fixed maturities and short term investments	(19,281,797)		(15,457,689)		(19,718,064)
Purchases of equity securities	(326,610)		(1,103,447)		(567,334)
Proceeds from sale of affiliates	593,539		397,091		748,575
Purchases of affiliates	(176,034)		(681,385)		(280,856)
Purchase of subsidiaries, net of acquired cash	0		(92,893)		(1,020,015)
Proceeds from sale of subsidiary	0		22,998		0
Returns of capital from subsidiaries	0		0		0
Change in restricted cash	(3,993)		1,488		(154,992)
Other, net	(72,550)		197,536		(155,019)
Net cash provided by (used in) investing activities	662,726		737,767		136,432
Proceeds from issuance of ordinary shares and exercise of stock options	50,634		9,854		9,976
Buybacks of ordinary shares	(571,614)		(1,051,050)		(468,971)
Repurchase of preference shares	(411,492)		0		0
Employee withholding on share-based compensation	(24,189)		(28,569)		(28,925)
Dividends paid on ordinary shares	(228,232)		(222,761)		(208,516)
Return of capital	0		0		0
Distributions to non-controlling interests	(124,305)		(130,447)		(117,683)
Contributions from non-controlling interest	59,583		47,524		23,610
Proceeds from issuance of debt	558,311		0		980,600
Repayment of debt	(17,909)		(8,248)		(87,447)
Deposit liabilities	(34,565)		(69,482)		(84,758)
Net cash provided by (used in) financing activities	(743,778)		(1,453,179)		17,886
Effects of exchange rate changes on foreign currency cash	56,699		(57,742)		(64,377)
Net change in cash and cash equivalents	8,966		170,752		734,422
Cash and cash equivalents – beginning of period	3,426,988		3,256,236		2,521,814
Cash and cash equivalents – end of period	3,435,954		3,426,988		3,256,236
Consolidating Adjustments and Eliminations
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(1,046,474)		(2,955,038)		(764,277)
Proceeds from sale of fixed maturities and short-term investments	0		(583,466)		(359,894)
Proceeds from redemption of fixed maturities and short-term investments	0		0		0
Proceeds from sale of equity securities	(50,256)		0		0
Purchases of fixed maturities and short term investments	0		583,466		359,894
Purchases of equity securities	45,582		0		0
Proceeds from sale of affiliates	0		0		0
Purchases of affiliates	0		0		0
Purchase of subsidiaries, net of acquired cash			0		2,317,699
Proceeds from sale of subsidiary			0		(2,317,699)
Returns of capital from subsidiaries	(462,178)		(739,930)		(2,112,037)
Change in restricted cash	0		0		0
Other, net	0		0		0
Net cash provided by (used in) investing activities	(466,852)		(739,930)		(2,112,037)
Proceeds from issuance of ordinary shares and exercise of stock options	0		0		0
Buybacks of ordinary shares	0		0		0
Repurchase of preference shares	4,674
Employee withholding on share-based compensation	0		0		0
Dividends paid on ordinary shares	1,045,987		2,955,038		764,277
Return of capital	462,178		739,930		2,112,037
Distributions to non-controlling interests	487		0		0
Contributions from non-controlling interest	0		0		0
Proceeds from issuance of debt	0				0
Repayment of debt	0		0		0
Deposit liabilities	0		0		0
Net cash provided by (used in) financing activities	1,513,326		3,694,968		2,876,314
Effects of exchange rate changes on foreign currency cash	0		0		0
Net change in cash and cash equivalents	0		0		0
Cash and cash equivalents – beginning of period	0		0		0
Cash and cash equivalents – end of period	0		0		0
XL-Group
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	284,709	[1]	978,695		561,468
Proceeds from sale of fixed maturities and short-term investments	0	[1]	0		0
Proceeds from redemption of fixed maturities and short-term investments	0	[1]	0		0
Proceeds from sale of equity securities	5,253	[1]	0		0
Purchases of fixed maturities and short term investments	0	[1]	0		0
Purchases of equity securities	0	[1]	0		0
Proceeds from sale of affiliates	0	[1]	0		0
Purchases of affiliates	0	[1]	0		0
Purchase of subsidiaries, net of acquired cash			0		(2,287,579)
Proceeds from sale of subsidiary			0		2,317,699
Returns of capital from subsidiaries	462,178	[1]	274,727		65,037
Change in restricted cash	0	[1]	0		0
Other, net	0	[1]	0		0
Net cash provided by (used in) investing activities	467,431	[1]	274,727		95,157
Proceeds from issuance of ordinary shares and exercise of stock options	50,634	[1]	9,854		9,976
Buybacks of ordinary shares	(571,614)	[1]	(1,051,050)		(468,971)
Repurchase of preference shares	0
Employee withholding on share-based compensation	0		0		0
Dividends paid on ordinary shares	(228,232)	[1]	(222,761)		(208,516)
Return of capital	0	[1]	0		0
Distributions to non-controlling interests	0	[1]	0		0
Contributions from non-controlling interest	0	[1]	0		0
Proceeds from issuance of debt	0				0
Repayment of debt	0	[1]	0		0
Deposit liabilities	0	[1]	0		0
Net cash provided by (used in) financing activities	(749,212)	[1]	(1,263,957)		(667,511)
Effects of exchange rate changes on foreign currency cash	0	[1]	0		0
Net change in cash and cash equivalents	2,928	[1]	(10,535)		(10,886)
Cash and cash equivalents – beginning of period	1,022	[1]	11,557		22,443
Cash and cash equivalents – end of period	3,950	[1]	1,022	[1]	11,557
XLIT
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	618,263		775,100		148,850
Proceeds from sale of fixed maturities and short-term investments	174,934		303,070		1,482,875
Proceeds from redemption of fixed maturities and short-term investments	137,658		97,168		188,788
Proceeds from sale of equity securities	45,680		0		0
Purchases of fixed maturities and short term investments	(208,395)		(534,502)		(1,296,980)
Purchases of equity securities	(45,582)		0		0
Proceeds from sale of affiliates	0		324		0
Purchases of affiliates	0		0		0
Purchase of subsidiaries, net of acquired cash			0		(2,317,699)
Proceeds from sale of subsidiary			0		0
Returns of capital from subsidiaries	0		465,203		2,047,000
Change in restricted cash	0		0		0
Other, net	1		0		0
Net cash provided by (used in) investing activities	104,296		331,263		103,984
Proceeds from issuance of ordinary shares and exercise of stock options	0		0		0
Buybacks of ordinary shares	0		0		0
Repurchase of preference shares	(371,163)
Employee withholding on share-based compensation	0		0		0
Dividends paid on ordinary shares	(295,987)		(1,039,253)		(764,277)
Return of capital	(462,178)		(274,727)		(65,037)
Distributions to non-controlling interests	(59,231)		(78,094)		(77,093)
Contributions from non-controlling interest	0		0		0
Proceeds from issuance of debt	558,311				980,600
Repayment of debt	(17,909)		0		0
Deposit liabilities	0		0		0
Net cash provided by (used in) financing activities	(648,157)		(1,392,074)		74,193
Effects of exchange rate changes on foreign currency cash	0		0		0
Net change in cash and cash equivalents	74,402		(285,711)		327,027
Cash and cash equivalents – beginning of period	84,286		369,997		42,970
Cash and cash equivalents – end of period	158,688		84,286		369,997
Other XL-Group Subsidiaries
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	176,821		2,145,149		698,440
Proceeds from sale of fixed maturities and short-term investments	15,527,192		13,037,976		15,745,247
Proceeds from redemption of fixed maturities and short-term investments	3,332,838		3,750,541		3,562,386
Proceeds from sale of equity securities	756,872		848,779		664,735
Purchases of fixed maturities and short term investments	(19,073,402)		(15,506,653)		(18,780,978)
Purchases of equity securities	(326,610)		(1,103,447)		(567,334)
Proceeds from sale of affiliates	593,539		396,767		748,575
Purchases of affiliates	(176,034)		(681,385)		(280,856)
Purchase of subsidiaries, net of acquired cash			(92,893)		1,267,564
Proceeds from sale of subsidiary			22,998		0
Returns of capital from subsidiaries	0		0		0
Change in restricted cash	(3,993)		1,488		(154,992)
Other, net	(72,551)		197,536		(155,019)
Net cash provided by (used in) investing activities	557,851		871,707		2,049,328
Proceeds from issuance of ordinary shares and exercise of stock options	0		0		0
Buybacks of ordinary shares	0		0		0
Repurchase of preference shares	(45,003)
Employee withholding on share-based compensation	(24,189)		(28,569)		(28,925)
Dividends paid on ordinary shares	(750,000)		(1,915,785)		0
Return of capital	0		(465,203)		(2,047,000)
Distributions to non-controlling interests	(65,561)		(52,353)		(40,590)
Contributions from non-controlling interest	59,583		47,524		23,610
Proceeds from issuance of debt	0				0
Repayment of debt	0		(8,248)		(87,447)
Deposit liabilities	(34,565)		(69,482)		(84,758)
Net cash provided by (used in) financing activities	(859,735)		(2,492,116)		(2,265,110)
Effects of exchange rate changes on foreign currency cash	56,699		(57,742)		(64,377)
Net change in cash and cash equivalents	(68,364)		466,998		418,281
Cash and cash equivalents – beginning of period	3,341,680		2,874,682		2,456,401
Cash and cash equivalents – end of period	$ 3,273,316		$ 3,341,680		$ 2,874,682

[1] Results for the period include XL-Ireland, which prior to the Redomestication was the ultimate parent and is now in liquidation proceedings as outlined in Note 1, "Significant Accounting Policies." The Company expects the net assets of XL-Ireland to ultimately be distributed to XL Group upon completion of the liquidation.