Guarantor Financial Information Income Statement (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenues:
Net premiums earned		$ 2,695,169	$ 2,516,917	$ 5,295,457	$ 5,039,708
Net investment income		231,788	208,674	450,269	409,206
Total realized investment gains (losses)		90,805	56,628	7,450	93,914
Net realized and unrealized gains (losses) on derivative instruments		16,126	(906)	20,347	(7,975)
Income (loss) from investment affiliates		16,548	30,818	62,217	69,079
Fee income and other		8,304	10,225	15,021	23,886
Total revenues		3,022,620	2,787,760	5,837,562	5,543,121
Expenses:
Net losses and loss expenses incurred		1,721,309	1,529,083	3,343,315	3,112,539
Claims and policy benefits		10,117	7,595	20,424	14,886
Acquisition costs		471,491	433,584	935,318	869,453	[1]
Operating expenses		470,508	472,778	943,071	940,816
Foreign exchange (gains) losses		(12,140)	(5,643)	(2,299)	(8,979)
Interest expense		48,686	54,018	102,231	104,729
Total expenses		2,709,971	2,491,415	5,342,060	5,033,444
Income (loss) before income tax expense and income (loss) from operating affiliate		312,649	296,345	495,502	509,677
Income (loss) from operating affiliates		51,187	42,704	61,469	56,313
Equity in net earnings (losses) of subsidiaries		0	0	0	0
Provision (benefit) for income tax		27,036	29,006	58,938	42,098
Net income (loss)		336,800	310,043	498,033	523,892
Non-controlling interests		17,804	8,423	26,389	69,429
Net income (loss) attributable to common shareholders		318,996	301,620	471,644	454,463
Comprehensive income (loss)		58,005	377,811	(148,570)	660,082
Life Funds Withheld Assets
Revenues:
Net investment income		30,252	31,439	60,650	64,803
Net realized and unrealized gains (losses) on derivative instruments		(36,120)	(34,596)	(13,199)	(84,697)
Consolidating Adjustments and Eliminations
Revenues:
Net premiums earned		0	0	0	0
Net investment income		0	(10)	0	(94)
Total realized investment gains (losses)		0	0	0	0
Net realized and unrealized gains (losses) on derivative instruments		0	0	0	0
Income (loss) from investment affiliates		0	0	0	0
Fee income and other		0	0	0	0
Total revenues		0	(10)	0	(94)
Expenses:
Net losses and loss expenses incurred		0	0	0	0
Claims and policy benefits		0	0	0	0
Acquisition costs		0	0	0	0
Operating expenses		0	0	0	0
Foreign exchange (gains) losses		0	0	0	0
Interest expense		0	0	0	0
Total expenses		0	0	0	0
Income (loss) before income tax expense and income (loss) from operating affiliate		0	(10)	0	(94)
Income (loss) from operating affiliates		0	0	0	0
Equity in net earnings (losses) of subsidiaries		(692,890)	(692,776)	(1,111,936)	(1,101,418)
Provision (benefit) for income tax		0	0	0	0
Net income (loss)		(692,890)	(692,786)	(1,111,936)	(1,101,512)
Non-controlling interests		0	(10)	0	(94)
Net income (loss) attributable to common shareholders		(692,890)	(692,776)	(1,111,936)	(1,101,418)
Comprehensive income (loss)		(170,908)	(845,158)	128,492	(1,512,656)
Consolidating Adjustments and Eliminations | Life Funds Withheld Assets
Revenues:
Net realized and unrealized gains (losses) on derivative instruments		0	0	0	0
XL-Group/XL-Ireland
Revenues:
Net premiums earned	[2]	0	0	0	0
Net investment income	[2]	39	10	47	94
Total realized investment gains (losses)	[2]	0	0	0	0
Net realized and unrealized gains (losses) on derivative instruments	[2]	0	0	0	0
Income (loss) from investment affiliates	[2]	0	0	0	0
Fee income and other	[2]	0	0	4	0
Total revenues	[2]	39	10	51	94
Expenses:
Net losses and loss expenses incurred	[2]	0	0	0	0
Claims and policy benefits	[2]	0	0	0	0
Acquisition costs	[2]	0	0	0	0
Operating expenses	[2]	5,765	27,389	36,627	43,570
Foreign exchange (gains) losses	[2]	(2,183)	936	(913)	1,112
Interest expense	[2]	0	0	0	0
Total expenses	[2]	3,582	28,325	35,714	44,682
Income (loss) before income tax expense and income (loss) from operating affiliate	[2]	(3,543)	(28,315)	(35,663)	(44,588)
Income (loss) from operating affiliates	[2]	0	0	0	0
Equity in net earnings (losses) of subsidiaries	[2]	322,628	329,935	507,396	499,051
Provision (benefit) for income tax	[2]	89	0	89	0
Net income (loss)	[2]	318,996	301,620	471,644	454,463
Non-controlling interests	[2]	0	0	0	0
Net income (loss) attributable to common shareholders	[2]	318,996	301,620	471,644	454,463
Comprehensive income (loss)	[2]	58,005	377,811	(148,570)	660,082
XL-Group/XL-Ireland | Life Funds Withheld Assets
Revenues:
Net realized and unrealized gains (losses) on derivative instruments	[2]	0	0	0	0
XLIT
Revenues:
Net premiums earned		0	0	0	0
Net investment income		3,565	2,103	6,852	4,416
Total realized investment gains (losses)		(689)	1,446	(2,275)	669
Net realized and unrealized gains (losses) on derivative instruments		0	0	0	0
Income (loss) from investment affiliates		0	0	0	0
Fee income and other		0	0	0	0
Total revenues		2,876	3,549	4,577	5,085
Expenses:
Net losses and loss expenses incurred		0	0	0	0
Claims and policy benefits		0	0	0	0
Acquisition costs		0	0	0	0
Operating expenses		945	1,152	1,787	1,661
Foreign exchange (gains) losses		(1,114)	26	(604)	27
Interest expense		40,206	35,277	80,245	70,424
Total expenses		40,037	36,455	81,428	72,112
Income (loss) before income tax expense and income (loss) from operating affiliate		(37,161)	(32,906)	(76,851)	(67,027)
Income (loss) from operating affiliates		0	0	0	0
Equity in net earnings (losses) of subsidiaries		370,262	362,841	604,540	602,367
Provision (benefit) for income tax		0	0	0	0
Net income (loss)		333,101	329,935	527,689	535,340
Non-controlling interests		10,473	0	20,293	36,289
Net income (loss) attributable to common shareholders		322,628	329,935	507,396	499,051
Comprehensive income (loss)		61,637	406,126	(112,818)	704,670
XLIT | Life Funds Withheld Assets
Revenues:
Net realized and unrealized gains (losses) on derivative instruments		0	0	0	0
Other XL Subsidiaries
Revenues:
Net premiums earned		2,695,169	2,516,917	5,295,457	5,039,708
Net investment income		228,184	206,571	443,370	404,790
Total realized investment gains (losses)		91,494	55,182	9,725	93,245
Net realized and unrealized gains (losses) on derivative instruments		16,126	(906)	20,347	(7,975)
Income (loss) from investment affiliates		16,548	30,818	62,217	69,079
Fee income and other		8,304	10,225	15,017	23,886
Total revenues		3,019,705	2,784,211	5,832,934	5,538,036
Expenses:
Net losses and loss expenses incurred		1,721,309	1,529,083	3,343,315	3,112,539
Claims and policy benefits		10,117	7,595	20,424	14,886
Acquisition costs		471,491	433,584	935,318	869,453
Operating expenses		463,798	444,237	904,657	895,585
Foreign exchange (gains) losses		(8,843)	(6,605)	(782)	(10,118)
Interest expense		8,480	18,741	21,986	34,305
Total expenses		2,666,352	2,426,635	5,224,918	4,916,650
Income (loss) before income tax expense and income (loss) from operating affiliate		353,353	357,576	608,016	621,386
Income (loss) from operating affiliates		51,187	42,704	61,469	56,313
Equity in net earnings (losses) of subsidiaries		0	0	0	0
Provision (benefit) for income tax		26,947	29,006	58,849	42,098
Net income (loss)		377,593	371,274	610,636	635,601
Non-controlling interests		7,331	8,433	6,096	33,234
Net income (loss) attributable to common shareholders		370,262	362,841	604,540	602,367
Comprehensive income (loss)		109,271	439,032	(15,674)	807,986
Other XL Subsidiaries | Life Funds Withheld Assets
Revenues:
Net realized and unrealized gains (losses) on derivative instruments		$ (36,120)	$ (34,596)	$ (13,199)	$ (84,697)

[1] The Company has reflected the amortization of certain fair value adjustments recorded in conjunction with the Catlin Acquisition within the respective segments

[2] 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.