Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Dec. 31, 2014
ASSETS
Total investments available for sale		$ 32,458,436		$ 31,919,126
Total investments trading		2,021,350		1,626,577
Investments in affiliates		1,911,996		2,177,645
Other investments		1,163,863		1,164,564
Total investments		37,555,645		36,887,912
Cash and cash equivalents		3,435,954		3,426,988		$ 3,256,236	$ 2,521,814
Restricted cash		157,497		153,504
Investments in subsidiaries on an equity basis		0		0
Accrued investment income		272,149		284,366
Deferred acquisition costs and value of business acquired		1,102,474		946,721
Ceded unearned premiums		2,198,217		1,687,864
Premiums receivable		6,934,482		5,522,976
Reinsurance balances receivable		930,114		577,479
Unpaid losses and loss expenses recoverable		7,247,723		5,491,297
Receivable from investments sold		201,515		128,411
Goodwill and other intangible assets		2,225,751		2,203,653		2,210,266	447,952
Deferred tax asset		332,024		310,542
Amounts due from subsidiaries		0		0
Other assets		842,691		812,389
Total assets		63,436,236		58,434,102
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses		29,696,779		25,939,571		25,439,744	19,353,243
Deposit liabilities		1,042,677		1,116,233
Future policy benefit reserves		3,610,926		3,506,047
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable		999,219		998,968
Unearned premiums		8,307,431		7,293,028
Notes payable and debt	[1]	3,220,769		2,647,677
Reinsurance balances payable		3,706,116		2,451,717
Payable for investments purchased		332,989		226,009
Deferred tax liability		57,574		77,271
Amounts due to subsidiaries		0		0
Other liabilities		1,000,436		1,216,902
Total liabilities		51,974,916		45,473,423
Shareholders' equity attributable to XL Group Ltd		9,848,317		10,938,512
Non-controlling interest in equity of consolidated subsidiaries		1,613,003		2,022,167
Total shareholders' equity		11,461,320		12,960,679		13,654,463
Total liabilities and shareholders' equity		63,436,236		58,434,102
Consolidating Adjustments and Eliminations
ASSETS
Total investments available for sale		0		0
Total investments trading		0		0
Investments in affiliates		0		0
Other investments		0		0
Total investments		0		0
Cash and cash equivalents		0		0		0	0
Restricted cash		0		0
Investments in subsidiaries on an equity basis		(23,197,062)		(25,135,893)
Accrued investment income		0		(55)
Deferred acquisition costs and value of business acquired		0		0
Ceded unearned premiums		0		0
Premiums receivable		0		0
Reinsurance balances receivable		0		0
Unpaid losses and loss expenses recoverable		0		0
Receivable from investments sold		0		0
Goodwill and other intangible assets		0		0
Deferred tax asset		0		0
Amounts due from subsidiaries		(94,277)		(115,742)
Other assets		0		0
Total assets		(23,291,339)		(25,251,690)
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses		0		0
Deposit liabilities		0		0
Future policy benefit reserves		0		0
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable		0		0
Unearned premiums		0		0
Notes payable and debt		0		0
Reinsurance balances payable		0		0
Payable for investments purchased		0		0
Deferred tax liability		0		0
Amounts due to subsidiaries		(94,277)		(115,742)
Other liabilities		0		0
Total liabilities		(94,277)		(115,742)
Shareholders' equity attributable to XL Group Ltd		(23,197,062)		(25,135,893)
Non-controlling interest in equity of consolidated subsidiaries		0		(55)
Total shareholders' equity		(23,197,062)		(25,135,948)
Total liabilities and shareholders' equity		(23,291,339)		(25,251,690)
XL-Group
ASSETS
Total investments available for sale		0	[2]	0
Total investments trading		0	[2]	0
Investments in affiliates		0	[2]	0
Other investments		0	[2]	0
Total investments		0	[2]	0
Cash and cash equivalents		3,950	[2]	1,022	[2]	11,557	22,443
Restricted cash		0	[2]	0
Investments in subsidiaries on an equity basis		9,817,979	[2]	10,897,195
Accrued investment income		11	[2]	55
Deferred acquisition costs and value of business acquired		0	[2]	0
Ceded unearned premiums		0	[2]	0
Premiums receivable		0	[2]	0
Reinsurance balances receivable		0	[2]	0
Unpaid losses and loss expenses recoverable		0	[2]	0
Receivable from investments sold		0	[2]	0
Goodwill and other intangible assets		0	[2]	0
Deferred tax asset		0	[2]	0
Amounts due from subsidiaries		32,301	[2]	42,780
Other assets		14,541	[2]	15,188
Total assets		9,868,782	[2]	10,956,240
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses		0	[2]	0
Deposit liabilities		0	[2]	0
Future policy benefit reserves		0	[2]	0
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable		0	[2]	0
Unearned premiums		0	[2]	0
Notes payable and debt		0	[2]	0
Reinsurance balances payable		0	[2]	0
Payable for investments purchased		0	[2]	0
Deferred tax liability		0	[2]	0
Amounts due to subsidiaries		0	[2]	0
Other liabilities		20,465	[2]	17,728
Total liabilities		20,465	[2]	17,728
Shareholders' equity attributable to XL Group Ltd		9,848,317	[2]	10,938,512
Non-controlling interest in equity of consolidated subsidiaries		0	[2]	0
Total shareholders' equity		9,848,317	[2]	10,938,512
Total liabilities and shareholders' equity		9,868,782	[2]	10,956,240
XLIT
ASSETS
Total investments available for sale		528,152		638,236
Total investments trading		0		0
Investments in affiliates		0		0
Other investments		0		9
Total investments		528,152		638,245
Cash and cash equivalents		158,688		84,286		369,997	42,970
Restricted cash		0		0
Investments in subsidiaries on an equity basis		13,379,083		14,238,698
Accrued investment income		2,436		2,981
Deferred acquisition costs and value of business acquired		0		0
Ceded unearned premiums		0		0
Premiums receivable		0		0
Reinsurance balances receivable		0		0
Unpaid losses and loss expenses recoverable		0		0
Receivable from investments sold		0		0
Goodwill and other intangible assets		0		0
Deferred tax asset		0		0
Amounts due from subsidiaries		0		0
Other assets		27,244		40,108
Total assets		14,095,603		15,004,318
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses		0		0
Deposit liabilities		0		0
Future policy benefit reserves		0		0
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable		0		0
Unearned premiums		0		0
Notes payable and debt		3,220,769		2,647,677
Reinsurance balances payable		0		0
Payable for investments purchased		848		0
Deferred tax liability		0		0
Amounts due to subsidiaries		62,111		72,962
Other liabilities		36,975		41,984
Total liabilities		3,320,703		2,762,623
Shareholders' equity attributable to XL Group Ltd		9,817,979		10,897,195
Non-controlling interest in equity of consolidated subsidiaries		956,921		1,344,500
Total shareholders' equity		10,774,900		12,241,695
Total liabilities and shareholders' equity		14,095,603		15,004,318
Other XL-Group Subsidiaries
ASSETS
Total investments available for sale		31,930,284		31,280,890
Total investments trading		2,021,350		1,626,577
Investments in affiliates		1,911,996		2,177,645
Other investments		1,163,863		1,164,555
Total investments		37,027,493		36,249,667
Cash and cash equivalents		3,273,316		3,341,680		$ 2,874,682	$ 2,456,401
Restricted cash		157,497		153,504
Investments in subsidiaries on an equity basis		0		0
Accrued investment income		269,702		281,385
Deferred acquisition costs and value of business acquired		1,102,474		946,721
Ceded unearned premiums		2,198,217		1,687,864
Premiums receivable		6,934,482		5,522,976
Reinsurance balances receivable		930,114		577,479
Unpaid losses and loss expenses recoverable		7,247,723		5,491,297
Receivable from investments sold		201,515		128,411
Goodwill and other intangible assets		2,225,751		2,203,653
Deferred tax asset		332,024		310,542
Amounts due from subsidiaries		61,976		72,962
Other assets		800,906		757,093
Total assets		62,763,190		57,725,234
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses		29,696,779		25,939,571
Deposit liabilities		1,042,677		1,116,233
Future policy benefit reserves		3,610,926		3,506,047
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable		999,219		998,968
Unearned premiums		8,307,431		7,293,028
Notes payable and debt		0		0
Reinsurance balances payable		3,706,116		2,451,717
Payable for investments purchased		332,141		226,009
Deferred tax liability		57,574		77,271
Amounts due to subsidiaries		32,166		42,780
Other liabilities		942,996		1,157,190
Total liabilities		48,728,025		42,808,814
Shareholders' equity attributable to XL Group Ltd		13,379,083		14,238,698
Non-controlling interest in equity of consolidated subsidiaries		656,082		677,722
Total shareholders' equity		14,035,165		14,916,420
Total liabilities and shareholders' equity		$ 62,763,190		$ 57,725,234

[1] "Outstanding" data represent December 31, 2017 and December 31, 2016 accreted values.

[2] 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.