Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) - USD ($) $ in Thousands			6 Months Ended
		Mar. 30, 2015	Jun. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 2,675,000	$ 2,675,000
Notes payable and debt			$ 2,646,324	2,644,970
Percentage ownership of subsidiaries			100.00%
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 300,000	300,000
Notes payable and debt	[2]		$ 298,350	298,015
Debt interest rate (percent)			2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 400,000	400,000
Notes payable and debt	[2]		$ 397,738	397,523
Debt interest rate (percent)			5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 350,000	350,000
Notes payable and debt	[2]		$ 349,084	349,029
Debt interest rate (percent)			6.375%
Senior notes | 6.25% Senior Notes due 2027
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 325,000	325,000
Notes payable and debt	[2]		$ 323,297	323,218
Debt interest rate (percent)			6.25%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 300,000	300,000
Notes payable and debt	[2]		$ 296,361	296,294
Debt interest rate (percent)			5.25%
Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 500,000	500,000
Notes payable and debt	[2]		$ 492,925	492,521
Debt interest rate (percent)			4.45%
Subordinated notes | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 500,000	500,000
Notes payable and debt	[2]		$ 488,569	$ 488,370
Debt interest rate (percent)			5.50%
XL Cayman | Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Debt interest rate (percent)		4.45%
Debt issue price, as a percentage of face value		99.633%
Net proceeds on issuance of debt		$ 492,200
Debt issuance cost		$ 5,900
XL Cayman | Subordinated notes | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Debt interest rate (percent)		5.50%
Debt issue price, as a percentage of face value		99.115%
Net proceeds on issuance of debt		$ 488,400
Debt issuance cost		$ 7,200
Allied International Holdings, Inc. | Trust Preferred Securities Subject to Mandatory Redemption [Member] | Trust Preferred Securities, Face $8 million, due 2035
Debt Instrument [Line Items]
Notes payable and debt			$ 8,248
Allied International Holdings, Inc. | Trust Preferred Securities Subject to Mandatory Redemption [Member] | Trust Preferred Securities, Face $8 million, due 2035 | 3 month LIBOR
Debt Instrument [Line Items]
Spread on variable rate			3.75%

[1] Excluded from the table are certain credit facilities under which the Company is permitted to utilize up to $1.4 billion at June 30, 2016 and December 31, 2015, respectively, for revolving loans to support general operating and financing needs. However, at June 30, 2016 and December 31, 2015, $501.2 million and $527.1 million, respectively, were utilized under these facilities to issue letters of credit, leaving $848.8 million and $822.9 million, respectively, available to support other operating and financing needs.

[2] "In Use/Outstanding" data represent June 30, 2016 and December 31, 2015 accreted values.