Statutory Financial Data (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2017 USD ($) subsidiary	Dec. 31, 2016 USD ($) subsidiary	Dec. 31, 2015 USD ($)
XL Group Ltd
Statutory Accounting Practices [Line Items]
Cash and investments, net of liabilities associated with cash sweeping arrangements		$ 4,000	$ 1,000
XL Cayman
Statutory Accounting Practices [Line Items]
Cash and investments, net of liabilities associated with cash sweeping arrangements		700,000	700,000
Bermuda
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	[1]	6,431,266	6,351,874
Actual statutory capital and surplus	[1],[2]	11,454,639	12,333,870
Statutory net income (loss)		$ (309,220)	724,711	$ 1,513,924
BSCR level for principal operating subsidiaries		100.00%
Maximum permitted dividends as a percentage of prior year statutory capital and surplus		25.00%
Maximum permitted dividends as a percentage of statutory capital		15.00%
Maximum dividend permitted without affidavit		$ 3,100,000	3,200,000
Distributable reserves, amount		7,800,000
United States
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	[3]	821,442	822,651
Actual statutory capital and surplus	[2],[3]	2,347,380	2,362,827
Statutory net income (loss)		$ (76,788)	96,481	17,574
RBC level for certain operating subsidiaries		100.00%
Maximum permitted dividends as a percentage of statutory policyholder surplus		10.00%
Maximum permitted dividends as a percentage of "Adjusted net investment income"		100.00%
United States | XL Reinsurance America, Inc. [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	[2],[3]	$ 2,100,000
Maximum dividend permitted without affidavit		0
Statutory earned surplus, amount		$ (138,900)	66,900
Number of property and casualty subsidiaries | subsidiary		7
United States | Catlin Specialty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	[2],[3]	$ 243,600
Maximum dividend permitted without affidavit		0
Statutory earned surplus, amount		$ (10,100)	3,400
Number of property and casualty subsidiaries | subsidiary		3
United States | T.H.E. Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus	[2],[3]	$ 53,300
Maximum dividend permitted without affidavit		0
Europe, UK, and other
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus		4,551,068	4,320,140
Actual statutory capital and surplus	[2]	5,711,162	5,201,602
Statutory net income (loss)		(335,422)	$ 366,708	$ (125,758)
Ireland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus		1,000,000
Funds at Lloyds [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus		1,900,000
United Kingdom
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus		1,200,000
Switzerland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus		$ 1,600,000
Statutory earned surplus deficit | United States | XL Reinsurance America, Inc. [Member]
Statutory Accounting Practices [Line Items]
Number of property and casualty subsidiaries | subsidiary		5	1
Statutory earned surplus deficit | United States | Catlin Specialty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Number of property and casualty subsidiaries | subsidiary		3

[1] Required statutory capital and surplus at December 31, 2017 represents 100% Bermuda Solvency Capital Requirement ("BSCR") level for the top Bermuda operating subsidiary, XL Bermuda Ltd, calculated on a consolidated basis (and therefore includes a BSCR requirement for all regions).

[2] Statutory assets in Bermuda include investments in other U.S. and international subsidiaries reported separately herein.

[3] Required statutory capital and surplus represents 100% Risk-Based Capital level for principal U.S. operating subsidiaries.