Guarantor Financial Information Cash Flows (Details) - USD ($) $ in Thousands		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		$ 183,814	$ 677,782
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		11,354,376	9,581,243
Proceeds from redemption of fixed maturities and short-term investments		2,794,515	3,010,158
Proceeds from sale of equity securities		456,889	411,330
Purchases of fixed maturities and short term investments		(14,456,346)	(11,557,045)
Purchases of equity securities		(314,752)	(611,086)
Proceeds from sale of affiliates		375,628	325,050
Purchases of affiliates		(119,835)	(556,548)
Purchase of subsidiaries, net of cash acquired		0	(92,893)
Proceeds from Divestiture of Interest in Consolidated Subsidiaries		0	19,144
Returns of capital from subsidiaries		0	0
Change in restricted cash		(6,123)	4,874
Other, net		(37,147)	193,392
Net cash provided by (used in) investing activities		47,205	727,619
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		49,640	5,735
Buybacks of common shares		(571,586)	(906,137)
Repurchase of preference shares		(411,492)	0
Employee withholding on share-based compensation		(24,156)	(28,171)
Dividends paid on common shares		(171,905)	(169,346)
Return of Capital		0	0
Distributions to non-controlling interests		(109,213)	(93,470)
Contributions from non-controlling interests		49,474	6,394
Proceeds from Issuance of Long-term Debt		558,311	0
Repayment of debt		17,909	8,248
Deposit liabilities		(34,407)	(63,488)
Net cash provided by (used in) financing activities		(683,243)	(1,256,731)
Effects of exchange rate changes on foreign currency cash		55,324	(11,629)
Increase (decrease) in cash and cash equivalents		(396,900)	137,041
Cash and cash equivalents beginning of period		3,426,988	3,256,236
Cash and cash equivalents end of period		3,030,088	3,393,277
Consolidating Adjustments and Eliminations
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		(1,046,474)	(2,315,848)
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		0	(583,466)
Proceeds from redemption of fixed maturities and short-term investments		0	0
Proceeds from sale of equity securities		(49,791)	0
Purchases of fixed maturities and short term investments		0	583,466
Purchases of equity securities		45,582	0
Proceeds from sale of affiliates		0	0
Purchases of affiliates		0	0
Purchase of subsidiaries, net of cash acquired			0
Proceeds from Divestiture of Interest in Consolidated Subsidiaries			0
Returns of capital from subsidiaries		(406,683)	(614,661)
Change in restricted cash		0	0
Other, net		0	0
Net cash provided by (used in) investing activities		(410,892)	(614,661)
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		0	0
Buybacks of common shares		0	0
Repurchase of preference shares		4,209
Employee withholding on share-based compensation		0	0
Dividends paid on common shares		1,045,987	2,315,848
Return of Capital		406,683	614,661
Distributions to non-controlling interests		487	0
Contributions from non-controlling interests		0	0
Proceeds from Issuance of Long-term Debt		0
Repayment of debt		0	0
Deposit liabilities		0	0
Net cash provided by (used in) financing activities		1,457,366	2,930,509
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash and cash equivalents		0	0
Cash and cash equivalents beginning of period		0	0
Cash and cash equivalents end of period		0	0
XL- Bermuda/Ireland
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities	[1]	284,889	921,608
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments	[1]	0	0
Proceeds from redemption of fixed maturities and short-term investments	[1]	0	0
Proceeds from sale of equity securities	[1]	4,788	0
Purchases of fixed maturities and short term investments	[1]	0	0
Purchases of equity securities	[1]	0	0
Proceeds from sale of affiliates	[1]	0	0
Purchases of affiliates	[1]	0	0
Purchase of subsidiaries, net of cash acquired	[1]		0
Proceeds from Divestiture of Interest in Consolidated Subsidiaries	[1]		0
Returns of capital from subsidiaries	[1]	406,683	149,458
Change in restricted cash	[1]	0	0
Other, net	[1]	0	0
Net cash provided by (used in) investing activities	[1]	411,471	149,458
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options	[1]	49,640	5,735
Buybacks of common shares	[1]	(571,586)	(906,137)
Repurchase of preference shares		0
Employee withholding on share-based compensation	[1]	0	0
Dividends paid on common shares	[1]	(171,905)	(169,346)
Return of Capital	[1]	0	0
Distributions to non-controlling interests	[1]	0	0
Contributions from non-controlling interests	[1]	0	0
Proceeds from Issuance of Long-term Debt		0
Repayment of debt	[1]	0	0
Deposit liabilities	[1]	0	0
Net cash provided by (used in) financing activities	[1]	(693,851)	(1,069,748)
Effects of exchange rate changes on foreign currency cash	[1]	0	0
Increase (decrease) in cash and cash equivalents	[1]	2,509	1,318
Cash and cash equivalents beginning of period	[1]	1,022	11,557
Cash and cash equivalents end of period	[1]	3,531	12,875
XLIT
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		651,559	230,877
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		164,364	300,452
Proceeds from redemption of fixed maturities and short-term investments		122,743	65,881
Proceeds from sale of equity securities		45,680	0
Purchases of fixed maturities and short term investments		(175,067)	(526,206)
Purchases of equity securities		(45,582)	0
Proceeds from sale of affiliates		0	0
Purchases of affiliates		0	0
Purchase of subsidiaries, net of cash acquired			0
Proceeds from Divestiture of Interest in Consolidated Subsidiaries			0
Returns of capital from subsidiaries		0	465,203
Change in restricted cash		0	0
Other, net		0	0
Net cash provided by (used in) investing activities		112,138	305,330
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		0	0
Buybacks of common shares		0	0
Repurchase of preference shares		(370,698)
Employee withholding on share-based compensation		0	0
Dividends paid on common shares		(295,987)	(700,063)
Return of Capital		(406,683)	(149,458)
Distributions to non-controlling interests		(49,648)	(42,188)
Contributions from non-controlling interests		0	0
Proceeds from Issuance of Long-term Debt		558,311
Repayment of debt		17,909	0
Deposit liabilities		0	0
Net cash provided by (used in) financing activities		(582,614)	(891,709)
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash and cash equivalents		181,083	(355,502)
Cash and cash equivalents beginning of period		84,286	369,997
Cash and cash equivalents end of period		265,369	14,495
Other XL Subsidiaries
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		293,840	1,841,145
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		11,190,012	9,864,257
Proceeds from redemption of fixed maturities and short-term investments		2,671,772	2,944,277
Proceeds from sale of equity securities		456,212	411,330
Purchases of fixed maturities and short term investments		(14,281,279)	(11,614,305)
Purchases of equity securities		(314,752)	(611,086)
Proceeds from sale of affiliates		375,628	325,050
Purchases of affiliates		(119,835)	(556,548)
Purchase of subsidiaries, net of cash acquired			(92,893)
Proceeds from Divestiture of Interest in Consolidated Subsidiaries			19,144
Returns of capital from subsidiaries		0	0
Change in restricted cash		(6,123)	4,874
Other, net		(37,147)	193,392
Net cash provided by (used in) investing activities		(65,512)	887,492
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		0	0
Buybacks of common shares		0	0
Repurchase of preference shares		(45,003)
Employee withholding on share-based compensation		(24,156)	(28,171)
Dividends paid on common shares		(750,000)	(1,615,785)
Return of Capital		0	(465,203)
Distributions to non-controlling interests		(60,052)	(51,282)
Contributions from non-controlling interests		49,474	6,394
Proceeds from Issuance of Long-term Debt		0
Repayment of debt			8,248
Deposit liabilities		(34,407)	(63,488)
Net cash provided by (used in) financing activities		(864,144)	(2,225,783)
Effects of exchange rate changes on foreign currency cash		55,324	(11,629)
Increase (decrease) in cash and cash equivalents		(580,492)	491,225
Cash and cash equivalents beginning of period		3,341,680	2,874,682
Cash and cash equivalents end of period		$ 2,761,188	$ 3,365,907

[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 be distributed to XL Group upon completion of the liquidation.