Guarantor Financial Information Cash Flows (Details) - USD ($) $ in Thousands		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		$ (611,617)	$ (121,942)
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		7,431,308	6,387,610
Proceeds from redemption of fixed maturities and short-term investments		1,440,191	2,084,182
Proceeds from sale of equity securities		84,068	228,914
Purchases of fixed maturities and short term investments		(8,781,847)	(8,719,024)
Purchases of equity securities		(44,954)	(277,329)
Proceeds from sale of affiliates		302,199	236,861
Purchases of affiliates		(103,222)	(51,891)
Returns of capital from subsidiaries		0	0
Other, net		(29,985)	5,127
Net cash provided by (used in) investing activities		297,758	(105,550)
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options		60,855	42,630
Buybacks of common shares		(2,057)	(450,629)
Employee withholding on share-based compensation		(16,696)	(23,957)
Dividends paid on common shares		(113,726)	(115,589)
Return of capital		0	0
Distributions to non-controlling interests		(45,094)	(73,708)
Contributions from non-controlling interests		1,924	25
Proceeds from the issuance of debt		0	558,311
Contingent consideration paid on business combination		(5,000)	(7,304)
Deposit liabilities		(119,113)	(11,150)
Net cash provided by (used in) financing activities		(238,907)	(81,371)
Effects of exchange rate changes on foreign currency cash		(10,118)	43,423
Increase (decrease) in cash, cash equivalents and restricted cash		(562,884)	(265,440)
Cash, cash equivalents and restricted cash - beginning of year		3,593,451	3,580,492
Cash, cash equivalents and restricted cash - end of period		3,030,567	3,315,052
Consolidating Adjustments and Eliminations
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		0	(695,987)
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		0	0
Proceeds from redemption of fixed maturities and short-term investments		0	0
Proceeds from sale of equity securities		0	0
Purchases of fixed maturities and short term investments		0	0
Purchases of equity securities		0	0
Proceeds from sale of affiliates		0	0
Purchases of affiliates		0	0
Returns of capital from subsidiaries		(351,669)	(237,013)
Other, net		0	0
Net cash provided by (used in) investing activities		(351,669)	(237,013)
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
Employee withholding on share-based compensation		0	0
Dividends paid on common shares		0	695,987
Return of capital		351,669	237,013
Distributions to non-controlling interests		0	0
Contributions from non-controlling interests		0	0
Proceeds from the issuance of debt			0
Contingent consideration paid on business combination		0	0
Deposit liabilities		0	0
Net cash provided by (used in) financing activities		351,669	933,000
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash, cash equivalents and restricted cash		0	0
Cash, cash equivalents and restricted cash - beginning of year		0	0
Cash, cash equivalents and restricted cash - end of period		0	0
XL-Group/XL-Ireland
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities	[1]	3,103	286,697
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]	0	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
Returns of capital from subsidiaries	[1]	51,669	237,013
Other, net	[1]	0	0
Net cash provided by (used in) investing activities	[1]	51,669	237,013
Cash flows provided by (used in) financing activities:
Proceeds from issuance of ordinary shares and exercise of stock options	[1]	60,855	42,630
Buybacks of common shares	[1]	(2,057)	(450,629)
Employee withholding on share-based compensation	[1]	0	0
Dividends paid on common shares	[1]	(113,726)	(115,589)
Return of capital	[1]	0	0
Distributions to non-controlling interests	[1]	0	0
Contributions from non-controlling interests	[1]	0	0
Proceeds from the issuance of debt	[1]		0
Contingent consideration paid on business combination		0	0
Deposit liabilities	[1]	0	0
Net cash provided by (used in) financing activities	[1]	(54,928)	(523,588)
Effects of exchange rate changes on foreign currency cash	[1]	0	0
Increase (decrease) in cash, cash equivalents and restricted cash	[1]	(156)	122
Cash, cash equivalents and restricted cash - beginning of year	[1]	3,950	1,022
Cash, cash equivalents and restricted cash - end of period	[1]	3,794	1,144
XLIT
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		(377,105)	332,788
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		162,290	81,662
Proceeds from redemption of fixed maturities and short-term investments		58,759	82,392
Proceeds from sale of equity securities		0	677
Purchases of fixed maturities and short term investments		(127,823)	(143,851)
Purchases of equity securities		0	0
Proceeds from sale of affiliates		0	0
Purchases of affiliates		0	0
Returns of capital from subsidiaries		300,000	0
Other, net		0	0
Net cash provided by (used in) investing activities		393,226	20,880
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
Employee withholding on share-based compensation		0	0
Dividends paid on common shares		0	(295,987)
Return of capital		(51,669)	(237,013)
Distributions to non-controlling interests		(20,293)	(39,867)
Contributions from non-controlling interests		0	0
Proceeds from the issuance of debt			558,311
Contingent consideration paid on business combination		0	0
Deposit liabilities		0	0
Net cash provided by (used in) financing activities		(71,962)	(14,556)
Effects of exchange rate changes on foreign currency cash		0	0
Increase (decrease) in cash, cash equivalents and restricted cash		(55,841)	339,112
Cash, cash equivalents and restricted cash - beginning of year		158,688	84,286
Cash, cash equivalents and restricted cash - end of period		102,847	423,398
Other XL Subsidiaries
Cash flows provided by (used in) operating activities:
Net cash provided by (used in) operating activities		(237,615)	(45,440)
Cash flows provided by (used in) investing activities:
Proceeds from sale of fixed maturities and short-term investments		7,269,018	6,305,948
Proceeds from redemption of fixed maturities and short-term investments		1,381,432	2,001,790
Proceeds from sale of equity securities		84,068	228,237
Purchases of fixed maturities and short term investments		(8,654,024)	(8,575,173)
Purchases of equity securities		(44,954)	(277,329)
Proceeds from sale of affiliates		302,199	236,861
Purchases of affiliates		(103,222)	(51,891)
Returns of capital from subsidiaries		0	0
Other, net		(29,985)	5,127
Net cash provided by (used in) investing activities		204,532	(126,430)
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
Employee withholding on share-based compensation		(16,696)	(23,957)
Dividends paid on common shares		0	(400,000)
Return of capital		(300,000)	0
Distributions to non-controlling interests		(24,801)	(33,841)
Contributions from non-controlling interests		1,924	25
Proceeds from the issuance of debt			0
Contingent consideration paid on business combination		(5,000)	(7,304)
Deposit liabilities		(119,113)	(11,150)
Net cash provided by (used in) financing activities		(463,686)	(476,227)
Effects of exchange rate changes on foreign currency cash		(10,118)	43,423
Increase (decrease) in cash, cash equivalents and restricted cash		(506,887)	(604,674)
Cash, cash equivalents and restricted cash - beginning of year		3,430,813	3,495,184
Cash, cash equivalents and restricted cash - end of period		$ 2,923,926	$ 2,890,510

[1] Includes XL-Ireland, which was the former ultimate parent and is now in liquidation proceedings. The Company expects the net assets of XL-Ireland to be distributed to XL Group upon completion of the liquidation.