Guarantor Financial Information Balance Sheet (Details) - USD ($) $ in Thousands		Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
ASSETS
Fixed maturities AFS		$ 30,656,628	$ 30,928,988
Short-term investments, at fair value		500,170	815,481
Fixed maturities, trading at fair value		2,084,251	2,006,385
Short-term investments, trading at fair value		7,425	14,965
Equity securities, at fair value		637,872	713,967
Investments in affiliates		1,900,105	1,911,996
Other investments		1,186,983	1,163,863
Investments		36,973,434	37,555,645
Cash and cash equivalents		3,484,763	3,435,954
Restricted cash		155,023	157,497
Investments in subsidiaries		0	0
Accrued investment income		265,701	272,149
Deferred acquisition costs and value of business acquired		1,317,797	1,102,474
Ceded unearned premiums		2,950,077	2,198,217
Premiums receivable		7,988,469	6,934,482
Reinsurance balances receivable		1,210,620	930,114
Unpaid losses and loss expenses recoverable		7,271,013	7,247,723
Receivable from investments sold		258,084	201,515
Goodwill and other intangible assets		2,230,506	2,225,751
Deferred tax asset		356,667	332,024
Amounts due from subsidiaries/parent		0	0
Other assets		875,809	842,691
Total assets		65,337,963	63,436,236
Liabilities:
Unpaid losses and loss expenses		29,701,568	29,696,779	$ 26,445,124	$ 25,939,571
Deposit liabilities		982,963	1,042,677
Future policy benefit reserves		3,680,958	3,610,926
Funds withheld on GreyCastle life retrocession arrangements		989,140	999,219
Unearned premiums		9,687,293	8,307,431
Notes payable and debt		3,240,461	3,220,769
Reinsurance balances payable		4,432,331	3,706,116
Payable for investments purchased		317,339	332,989
Deferred tax liability		57,995	57,574
Amounts due to subsidiaries/parent		0	0
Other liabilities		1,012,693	1,000,436
Total liabilities		54,102,741	51,974,916
Shareholders' Equity:
Shareholders' equity attributable to XL Group Ltd		9,628,529	9,848,317
Non-controlling interest in equity of consolidated subsidiaries		1,606,693	1,613,003
Total shareholders' equity		11,235,222	11,461,320	$ 13,000,626
Total liabilities and shareholders' equity		65,337,963	63,436,236
Consolidating Adjustments and Eliminations
ASSETS
Fixed maturities AFS		0	0
Short-term investments, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Short-term investments, trading at fair value		0	0
Equity securities, at fair value		0	0
Investments in affiliates		0	0
Other investments		0	0
Investments		0	0
Cash and cash equivalents		0	0
Restricted cash		0	0
Investments in subsidiaries		(22,910,239)	(23,197,062)
Accrued investment income		0	0
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/parent		(128,214)	(94,277)
Other assets		0	0
Total assets		(23,038,453)	(23,291,339)
Liabilities:
Unpaid losses and loss expenses		0	0
Deposit liabilities		0	0
Future policy benefit reserves		0	0
Funds withheld on GreyCastle life retrocession arrangements		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/parent		(128,214)	(94,277)
Other liabilities		0	0
Total liabilities		(128,214)	(94,277)
Shareholders' Equity:
Shareholders' equity attributable to XL Group Ltd		(22,910,239)	(23,197,062)
Non-controlling interest in equity of consolidated subsidiaries		0	0
Total shareholders' equity		(22,910,239)	(23,197,062)
Total liabilities and shareholders' equity		(23,038,453)	(23,291,339)
XL-Group/XL-Ireland
ASSETS
Fixed maturities AFS	[1]	0	0
Short-term investments, at fair value	[1]	0	0
Fixed maturities, trading at fair value	[1]	0	0
Short-term investments, trading at fair value	[1]	0	0
Equity securities, at fair value	[1]	0	0
Investments in affiliates	[1]	0	0
Other investments	[1]	0	0
Investments	[1]	0	0
Cash and cash equivalents	[1]	20,360	3,950
Restricted cash	[1]	0	0
Investments in subsidiaries	[1]	9,632,524	9,817,979
Accrued investment income	[1]	11	11
Deferred acquisition costs and value of business acquired	[1]	0	0
Ceded unearned premiums	[1]	0	0
Premiums receivable	[1]	0	0
Reinsurance balances receivable	[1]	0	0
Unpaid losses and loss expenses recoverable	[1]	0	0
Receivable from investments sold	[1]	0	0
Goodwill and other intangible assets	[1]	0	0
Deferred tax asset	[1]	0	0
Amounts due from subsidiaries/parent	[1]	57,632	32,301
Other assets	[1]	1,916	14,541
Total assets	[1]	9,712,443	9,868,782
Liabilities:
Unpaid losses and loss expenses	[1]	0	0
Deposit liabilities	[1]	0	0
Future policy benefit reserves	[1]	0	0
Funds withheld on GreyCastle life retrocession arrangements	[1]	0	0
Unearned premiums	[1]	0	0
Notes payable and debt	[1]	0	0
Reinsurance balances payable	[1]	0	0
Payable for investments purchased	[1]	0	0
Deferred tax liability	[1]	0	0
Amounts due to subsidiaries/parent	[1]	0	0
Other liabilities	[1]	83,914	20,465
Total liabilities	[1]	83,914	20,465
Shareholders' Equity:
Shareholders' equity attributable to XL Group Ltd	[1]	9,628,529	9,848,317
Non-controlling interest in equity of consolidated subsidiaries	[1]	0	0
Total shareholders' equity	[1]	9,628,529	9,848,317
Total liabilities and shareholders' equity	[1]	9,712,443	9,868,782
XLIT
ASSETS
Fixed maturities AFS		449,368	528,152
Short-term investments, at fair value		0	0
Fixed maturities, trading at fair value		0	0
Short-term investments, trading at fair value		0	0
Equity securities, at fair value		0	0
Investments in affiliates		0	0
Other investments		0	0
Investments		449,368	528,152
Cash and cash equivalents		233,948	158,688
Restricted cash		0	0
Investments in subsidiaries		13,277,715	13,379,083
Accrued investment income		2,133	2,436
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		15,000	0
Goodwill and other intangible assets		0	0
Deferred tax asset		0	0
Amounts due from subsidiaries/parent		0	0
Other assets		27,611	27,244
Total assets		14,005,775	14,095,603
Liabilities:
Unpaid losses and loss expenses		0	0
Deposit liabilities		0	0
Future policy benefit reserves		0	0
Funds withheld on GreyCastle life retrocession arrangements		0	0
Unearned premiums		0	0
Notes payable and debt		3,240,461	3,220,769
Reinsurance balances payable		0	0
Payable for investments purchased		0	848
Deferred tax liability		1,949	0
Amounts due to subsidiaries/parent		122,328	62,111
Other liabilities		51,592	36,975
Total liabilities		3,416,330	3,320,703
Shareholders' Equity:
Shareholders' equity attributable to XL Group Ltd		9,632,524	9,817,979
Non-controlling interest in equity of consolidated subsidiaries		956,921	956,921
Total shareholders' equity		10,589,445	10,774,900
Total liabilities and shareholders' equity		14,005,775	14,095,603
Other XL Subsidiaries
ASSETS
Fixed maturities AFS		30,207,260	30,400,836
Short-term investments, at fair value		500,170	815,481
Fixed maturities, trading at fair value		2,084,251	2,006,385
Short-term investments, trading at fair value		7,425	14,965
Equity securities, at fair value		637,872	713,967
Investments in affiliates		1,900,105	1,911,996
Other investments		1,186,983	1,163,863
Investments		36,524,066	37,027,493
Cash and cash equivalents		3,230,455	3,273,316
Restricted cash		155,023	157,497
Investments in subsidiaries		0	0
Accrued investment income		263,557	269,702
Deferred acquisition costs and value of business acquired		1,317,797	1,102,474
Ceded unearned premiums		2,950,077	2,198,217
Premiums receivable		7,988,469	6,934,482
Reinsurance balances receivable		1,210,620	930,114
Unpaid losses and loss expenses recoverable		7,271,013	7,247,723
Receivable from investments sold		243,084	201,515
Goodwill and other intangible assets		2,230,506	2,225,751
Deferred tax asset		356,667	332,024
Amounts due from subsidiaries/parent		70,582	61,976
Other assets		846,282	800,906
Total assets		64,658,198	62,763,190
Liabilities:
Unpaid losses and loss expenses		29,701,568	29,696,779
Deposit liabilities		982,963	1,042,677
Future policy benefit reserves		3,680,958	3,610,926
Funds withheld on GreyCastle life retrocession arrangements		989,140	999,219
Unearned premiums		9,687,293	8,307,431
Notes payable and debt		0	0
Reinsurance balances payable		4,432,331	3,706,116
Payable for investments purchased		317,339	332,141
Deferred tax liability		56,046	57,574
Amounts due to subsidiaries/parent		5,886	32,166
Other liabilities		877,187	942,996
Total liabilities		50,730,711	48,728,025
Shareholders' Equity:
Shareholders' equity attributable to XL Group Ltd		13,277,715	13,379,083
Non-controlling interest in equity of consolidated subsidiaries		649,772	656,082
Total shareholders' equity		13,927,487	14,035,165
Total liabilities and shareholders' equity		$ 64,658,198	$ 62,763,190

[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.