Statutory Financial Data (Details) $ in Thousands		12 Months Ended
		Dec. 31, 2016 USD ($) subsidiary	Dec. 31, 2015 USD ($) subsidiary	Dec. 31, 2014 USD ($)
XL Group Ltd
Statutory Accounting Practices [Line Items]
Cash and investments, net of liabilities associated with cash sweeping arrangements		$ 1,000	$ 11,600
XL Cayman
Statutory Accounting Practices [Line Items]
Cash and investments, net of liabilities associated with cash sweeping arrangements		700,000	900,000
Bermuda
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	[1]	6,351,874	7,722,123
Actual statutory capital and surplus	[1],[2]	12,333,870	11,112,176
Statutory net income (loss)		$ 724,711	1,513,924	$ 361,022
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		$ 3,200,000	2,800,000
Distributable reserves, amount		8,100,000
United States
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus	[3]	822,651	747,313
Actual statutory capital and surplus	[2],[3]	2,362,827	2,234,767
Statutory net income (loss)		$ 96,481	17,574	155,751
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		66,900
Statutory earned surplus, amount		$ 66,900	31,700
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]	$ 234,200
Maximum dividend permitted without affidavit		0
Statutory earned surplus, amount		$ 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]	$ 52,300
Maximum dividend permitted without affidavit		0
Europe, UK, and other
Statutory Accounting Practices [Line Items]
Required statutory capital and surplus		4,320,140	4,445,247
Actual statutory capital and surplus	[2]	5,201,602	6,054,122
Statutory net income (loss)		366,708	$ (125,758)	$ 215,787
Ireland
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus		900,000
Funds at Lloyds [Member]
Statutory Accounting Practices [Line Items]
Actual statutory capital and surplus		1,500,000
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		$ 1,500,000
Statutory earned surplus deficit | United States | XL Reinsurance America, Inc. [Member]
Statutory Accounting Practices [Line Items]
Number of property and casualty subsidiaries | subsidiary		1	2
Statutory earned surplus deficit | United States | Catlin Specialty Insurance Company [Member]
Statutory Accounting Practices [Line Items]
Number of property and casualty subsidiaries | subsidiary		2

[1] Required statutory capital and surplus at December 31, 2016 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.