Guarantor Financial Information - Balance Sheet (Details) - USD ($) $ in Thousands	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014	Dec. 31, 2013
ASSETS
Total investments available for sale	$ 31,919,126		$ 33,753,898
Total investments trading	1,626,577		1,296,029
Investments in affiliates	2,177,645		1,708,899
Other investments	1,164,564		1,433,057
Total investments	36,887,912		38,191,883
Cash and cash equivalents	3,426,988		3,256,236		$ 2,521,814	$ 1,800,832
Restricted cash	153,504		154,992
Investments in subsidiaries on an equity basis	0		0
Accrued investment income	284,366		312,667
Deferred acquisition costs and value of business acquired	946,721		890,568
Ceded unearned premiums	1,687,864		1,821,793
Premiums receivable	5,522,976		4,712,493
Reinsurance balances receivable	577,479		418,666
Unpaid losses and loss expenses recoverable	5,491,297		5,262,706
Receivable from investments sold	128,411		231,158
Goodwill and other intangible assets	2,203,653		2,210,266		447,952	411,611
Deferred tax asset	310,542		282,311
Amounts due from subsidiaries	0		0
Other assets	812,389		937,199
Total assets	58,434,102		58,682,938
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses	25,939,571		25,439,744		19,353,243	20,481,065
Deposit liabilities	1,116,233		1,168,376
Future policy benefit reserves	3,506,047		4,163,500
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2016 - $3,072,615; 2015 - $3,719,131)	998,968		914,629
Unearned premiums	7,293,028		7,043,358
Notes payable and debt	2,647,677		2,644,970
Reinsurance balances payable	2,451,717		2,117,727
Payable for investments purchased	226,009		130,060
Deferred tax liability	77,271		120,651
Amounts due to subsidiaries	0		0
Other liabilities	1,216,902		1,285,460
Total liabilities	45,473,423		45,028,475
Shareholders' equity attributable to XL Group Ltd	10,938,512		11,677,079
Non-controlling interest in equity of consolidated subsidiaries	2,022,167		1,977,384
Total shareholders' equity	12,960,679		13,654,463		11,435,766
Total liabilities and shareholders' equity	58,434,102		58,682,938
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	(25,135,893)		(27,485,324)
Accrued investment income	(55)		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	(115,742)		(1,087,594)
Other assets	0		0
Total assets	(25,251,690)		(28,572,918)
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: 2016 - $3,072,615; 2015 - $3,719,131)	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	(115,742)		(1,087,594)
Other liabilities	0		0
Total liabilities	(115,742)		(1,087,594)
Shareholders' equity attributable to XL Group Ltd	(25,135,893)		(27,485,324)
Non-controlling interest in equity of consolidated subsidiaries	(55)		0
Total shareholders' equity	(25,135,948)		(27,485,324)
Total liabilities and shareholders' equity	(25,251,690)		(28,572,918)
XL-Bermuda
ASSETS
Total investments available for sale	0	[1]	0
Total investments trading	0	[1]	0
Investments in affiliates	0	[1]	0
Other investments	0	[1]	0
Total investments	0	[1]	0
Cash and cash equivalents	1,022	[1]	11,557	[1]	22,443	12,709
Restricted cash	0	[1]	0
Investments in subsidiaries on an equity basis	10,897,195	[1]	11,648,673
Accrued investment income	55	[1]	0
Deferred acquisition costs and value of business acquired	0	[1]	0
Ceded unearned premiums	0	[1]	0
Premiums receivable	0	[1]	0
Reinsurance balances receivable	0	[1]	0
Unpaid losses and loss expenses recoverable	0	[1]	0
Receivable from investments sold	0	[1]	0
Goodwill and other intangible assets	0	[1]	0
Deferred tax asset	0	[1]	0
Amounts due from subsidiaries	42,780	[1]	33,417
Other assets	15,188	[1]	2,748
Total assets	10,956,240	[1]	11,696,395
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses	0	[1]	0
Deposit liabilities	0	[1]	0
Future policy benefit reserves	0	[1]	0
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2016 - $3,072,615; 2015 - $3,719,131)	0	[1]	0
Unearned premiums	0	[1]	0
Notes payable and debt	0	[1]	0
Reinsurance balances payable	0	[1]	0
Payable for investments purchased	0	[1]	0
Deferred tax liability	0	[1]	0
Amounts due to subsidiaries	0	[1]	0
Other liabilities	17,728	[1]	19,316
Total liabilities	17,728	[1]	19,316
Shareholders' equity attributable to XL Group Ltd	10,938,512	[1]	11,677,079
Non-controlling interest in equity of consolidated subsidiaries	0	[1]	0
Total shareholders' equity	10,938,512	[1]	11,677,079
Total liabilities and shareholders' equity	10,956,240	[1]	11,696,395
XL-Cayman
ASSETS
Total investments available for sale	638,236		516,425
Total investments trading	0		0
Investments in affiliates	0		0
Other investments	9		877
Total investments	638,245		517,302
Cash and cash equivalents	84,286		369,997		42,970	35,794
Restricted cash	0		0
Investments in subsidiaries on an equity basis	14,238,698		15,836,651
Accrued investment income	2,981		2,323
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	40,108		44,570
Total assets	15,004,318		16,770,843
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: 2016 - $3,072,615; 2015 - $3,719,131)	0		0
Unearned premiums	0		0
Notes payable and debt	2,647,677		2,644,970
Reinsurance balances payable	0		0
Payable for investments purchased	0		0
Deferred tax liability	0		0
Amounts due to subsidiaries	72,962		1,087,594
Other liabilities	41,984		45,106
Total liabilities	2,762,623		3,777,670
Shareholders' equity attributable to XL Group Ltd	10,897,195		11,648,673
Non-controlling interest in equity of consolidated subsidiaries	1,344,500		1,344,500
Total shareholders' equity	12,241,695		12,993,173
Total liabilities and shareholders' equity	15,004,318		16,770,843
Other XL-Bermuda Subsidiaries
ASSETS
Total investments available for sale	31,280,890		33,237,473
Total investments trading	1,626,577		1,296,029
Investments in affiliates	2,177,645		1,708,899
Other investments	1,164,555		1,432,180
Total investments	36,249,667		37,674,581
Cash and cash equivalents	3,341,680		2,874,682		$ 2,456,401	$ 1,752,329
Restricted cash	153,504		154,992
Investments in subsidiaries on an equity basis	0		0
Accrued investment income	281,385		310,344
Deferred acquisition costs and value of business acquired	946,721		890,568
Ceded unearned premiums	1,687,864		1,821,793
Premiums receivable	5,522,976		4,712,493
Reinsurance balances receivable	577,479		418,666
Unpaid losses and loss expenses recoverable	5,491,297		5,262,706
Receivable from investments sold	128,411		231,158
Goodwill and other intangible assets	2,203,653		2,210,266
Deferred tax asset	310,542		282,311
Amounts due from subsidiaries	72,962		1,054,177
Other assets	757,093		889,881
Total assets	57,725,234		58,788,618
LIABILITIES AND SHAREHOLDERS' EQUITY
Unpaid losses and loss expenses	25,939,571		25,439,744
Deposit liabilities	1,116,233		1,168,376
Future policy benefit reserves	3,506,047		4,163,500
Funds withheld on GreyCastle life retrocession arrangements (net of future policy benefit reserves recoverable: 2016 - $3,072,615; 2015 - $3,719,131)	998,968		914,629
Unearned premiums	7,293,028		7,043,358
Notes payable and debt	0		0
Reinsurance balances payable	2,451,717		2,117,727
Payable for investments purchased	226,009		130,060
Deferred tax liability	77,271		120,651
Amounts due to subsidiaries	42,780		0
Other liabilities	1,157,190		1,221,038
Total liabilities	42,808,814		42,319,083
Shareholders' equity attributable to XL Group Ltd	14,238,698		15,836,651
Non-controlling interest in equity of consolidated subsidiaries	677,722		632,884
Total shareholders' equity	14,916,420		16,469,535
Total liabilities and shareholders' equity	$ 57,725,234		$ 58,788,618

[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.