Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) - USD ($) $ in Thousands			9 Months Ended
		Mar. 30, 2015	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Commitment, debt	[1]		$ 2,675,000	$ 2,675,000
Notes payable and debt			$ 2,647,000	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]		$ 298,518	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]		$ 397,846	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]		$ 349,111	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]		$ 323,336	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]		$ 296,394	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]		$ 493,127	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]		$ 488,668	$ 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			$ 8,200
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 $1.1 billion and $1.4 billion as of September 30, 2016 and December 31, 2015, respectively, for revolving loans to support general operating and financing needs. However, as of September 30, 2016 and December 31, 2015, $501.5 million and $527.1 million, respectively, were utilized under these facilities to issue letters of credit, leaving $598.5 million and $822.9 million, respectively, available to support other operating and financing needs.

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