Guarantor Financial Information - Cash Flow (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	$ 915,337		$ 615,556		$ 963,184
Proceeds from sale of fixed maturities and short-term investments	12,757,580		16,868,228		4,896,112
Proceeds from redemption of fixed maturities and short-term investments	3,847,709		3,751,174		3,601,793
Proceeds from sale of equity securities	848,779		664,735		571,410
Purchases of fixed maturities and short term investments	(15,457,689)		(19,718,064)		(7,158,258)
Purchases of equity securities	(1,103,447)		(567,334)		(445,504)
Proceeds from sale of affiliates	397,091		748,575		240,785
Purchases of affiliates	(681,385)		(280,856)		(371,226)
Purchase of subsidiaries, net of acquired cash	(92,893)		(1,020,015)		0
Proceeds from sale of subsidiary	22,998		0		570,000
Returns of capital from subsidiaries	0		0		0
Change in restricted cash	1,488		(154,992)		0
Other, net	197,536		(155,019)		(193,491)
Net cash provided by (used in) investing activities	737,767		136,432		1,711,621
Proceeds from issuance of ordinary shares and exercise of stock options	9,854		9,976		6,367
Buybacks of ordinary shares	(1,051,050)		(468,971)		(801,953)
Dividends paid on ordinary shares	(222,761)		(208,516)		(169,620)
Return of capital	0		0		0
Distributions to non-controlling interests	(130,447)		(117,683)		(78,465)
Contributions from non-controlling interest	47,524		23,610		48,261
Proceeds from issuance of debt	0		980,600		0
Repayment of debt	(8,248)		(87,447)		(600,000)
Net proceeds from affiliated notional cash pooling programs			0		0
Deposit liabilities	(69,482)		(84,758)		(268,298)
Net cash provided by (used in) financing activities	(1,424,610)		46,811		(1,863,708)
Effects of exchange rate changes on foreign currency cash	(57,742)		(64,377)		(90,115)
Net change in cash and cash equivalents	170,752		734,422		720,982
Cash and cash equivalents – beginning of period	3,256,236		2,521,814		1,800,832
Cash and cash equivalents – end of period	3,426,988		3,256,236		2,521,814
Consolidating Adjustments and Eliminations
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	(2,955,038)		(764,277)		(858,521)
Proceeds from sale of fixed maturities and short-term investments	(583,466)		(359,894)		(472,539)
Proceeds from redemption of fixed maturities and short-term investments	0		0		0
Proceeds from sale of equity securities	0		0		0
Purchases of fixed maturities and short term investments	583,466		359,894		472,539
Purchases of equity securities	0		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)		0
Returns of capital from subsidiaries	(739,930)		(2,112,037)		(561,656)
Change in restricted cash	0		0
Other, net	0		0		0
Net cash provided by (used in) investing activities	(739,930)		(2,112,037)		(561,656)
Proceeds from issuance of ordinary shares and exercise of stock options	0		0		0
Buybacks of ordinary shares	0		0		0
Dividends paid on ordinary shares	2,955,038		764,277		858,521
Return of capital	739,930		2,112,037		561,656
Distributions to non-controlling interests	0		0		0
Contributions from non-controlling interest	0		0		0
Proceeds from issuance of debt			0
Repayment of debt	0		0		0
Net proceeds from affiliated notional cash pooling programs			0		0
Deposit liabilities	0		0		0
Net cash provided by (used in) financing activities	3,694,968		2,876,314		1,420,177
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-Bermuda
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	978,695	[1]	561,468		895,854
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	0	[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	[1]	(2,287,579)
Proceeds from sale of subsidiary	0	[1]	2,317,699		0
Returns of capital from subsidiaries	274,727	[1]	65,037		79,086
Change in restricted cash	0	[1]	0
Other, net	0	[1]	0		0
Net cash provided by (used in) investing activities	274,727	[1]	95,157		79,086
Proceeds from issuance of ordinary shares and exercise of stock options	9,854	[1]	9,976		6,367
Buybacks of ordinary shares	(1,051,050)	[1]	(468,971)		(801,953)
Dividends paid on ordinary shares	(222,761)	[1]	(208,516)		(169,620)
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
Repayment of debt	0	[1]	0		0
Net proceeds from affiliated notional cash pooling programs			0		0
Deposit liabilities	0	[1]	0		0
Net cash provided by (used in) financing activities	(1,263,957)	[1]	(667,511)		(965,206)
Effects of exchange rate changes on foreign currency cash	0	[1]	0		0
Net change in cash and cash equivalents	(10,535)	[1]	(10,886)		9,734
Cash and cash equivalents – beginning of period	11,557	[1]	22,443		12,709
Cash and cash equivalents – end of period	1,022	[1]	11,557	[1]	22,443
XL-Cayman
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	775,100		148,850		518,715
Proceeds from sale of fixed maturities and short-term investments	303,070		1,482,875		574,673
Proceeds from redemption of fixed maturities and short-term investments	97,168		188,788		351,737
Proceeds from sale of equity securities	0		0		0
Purchases of fixed maturities and short term investments	(534,502)		(1,296,980)		(307,830)
Purchases of equity securities	0		0		0
Proceeds from sale of affiliates	324		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		0		0
Returns of capital from subsidiaries	465,203		2,047,000		482,570
Change in restricted cash	0		0
Other, net	0		0		1,920
Net cash provided by (used in) investing activities	331,263		103,984		1,103,070
Proceeds from issuance of ordinary shares and exercise of stock options	0		0		0
Buybacks of ordinary shares	0		0		0
Dividends paid on ordinary shares	(1,039,253)		(764,277)		(858,521)
Return of capital	(274,727)		(65,037)		(79,086)
Distributions to non-controlling interests	(78,094)		(77,093)		(77,002)
Contributions from non-controlling interest	0		0		0
Proceeds from issuance of debt			980,600
Repayment of debt	0		0		(600,000)
Net proceeds from affiliated notional cash pooling programs			0		0
Deposit liabilities	0		0		0
Net cash provided by (used in) financing activities	(1,392,074)		74,193		(1,614,609)
Effects of exchange rate changes on foreign currency cash	0		0		0
Net change in cash and cash equivalents	(285,711)		327,027		7,176
Cash and cash equivalents – beginning of period	369,997		42,970		35,794
Cash and cash equivalents – end of period	84,286		369,997		42,970
Other XL-Bermuda Subsidiaries
Condensed Cash Flow Statements, Captions [Line Items]
Net cash provided by (used in) operating activities	2,116,580		669,515		407,136
Proceeds from sale of fixed maturities and short-term investments	13,037,976		15,745,247		4,793,978
Proceeds from redemption of fixed maturities and short-term investments	3,750,541		3,562,386		3,250,056
Proceeds from sale of equity securities	848,779		664,735		571,410
Purchases of fixed maturities and short term investments	(15,506,653)		(18,780,978)		(7,322,967)
Purchases of equity securities	(1,103,447)		(567,334)		(445,504)
Proceeds from sale of affiliates	396,767		748,575		240,785
Purchases of affiliates	(681,385)		(280,856)		(371,226)
Purchase of subsidiaries, net of acquired cash	(92,893)		1,267,564
Proceeds from sale of subsidiary	22,998		0		570,000
Returns of capital from subsidiaries	0		0		0
Change in restricted cash	1,488		(154,992)
Other, net	197,536		(155,019)		(195,411)
Net cash provided by (used in) investing activities	871,707		2,049,328		1,091,121
Proceeds from issuance of ordinary shares and exercise of stock options	0		0		0
Buybacks of ordinary shares	0		0		0
Dividends paid on ordinary shares	(1,915,785)		0		0
Return of capital	(465,203)		(2,047,000)		(482,570)
Distributions to non-controlling interests	(52,353)		(40,590)		(1,463)
Contributions from non-controlling interest	47,524		23,610		48,261
Proceeds from issuance of debt			0
Repayment of debt	(8,248)		(87,447)		0
Net proceeds from affiliated notional cash pooling programs			0		0
Deposit liabilities	(69,482)		(84,758)		(268,298)
Net cash provided by (used in) financing activities	(2,463,547)		(2,236,185)		(704,070)
Effects of exchange rate changes on foreign currency cash	(57,742)		(64,377)		(90,115)
Net change in cash and cash equivalents	466,998		418,281		704,072
Cash and cash equivalents – beginning of period	2,874,682		2,456,401		1,752,329
Cash and cash equivalents – end of period	$ 3,341,680		$ 2,874,682		$ 2,456,401

[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-Bermuda upon completion of the liquidation.