Notes Payable and Debt and Financing Arrangements - Notes payable and debt (Details) - USD ($) $ in Thousands		3 Months Ended
	Mar. 30, 2015	Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Commitment, debt		$ 2,683,248	$ 2,675,000
Notes payable and debt		$ 2,653,895	2,644,970
Percentage ownership of subsidiaries		100.00%
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Commitment, debt		$ 300,000	300,000
Notes payable and debt		$ 298,183	298,015
Debt interest rate (percent)		2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Commitment, debt		$ 400,000	400,000
Notes payable and debt		$ 397,630	397,523
Debt interest rate (percent)		5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Commitment, debt		$ 350,000	350,000
Notes payable and debt		$ 349,057	349,029
Debt interest rate (percent)		6.375%
Senior notes | 6.25% Senior Notes due 2027
Debt Instrument [Line Items]
Commitment, debt		$ 325,000	325,000
Notes payable and debt		$ 323,257	323,218
Debt interest rate (percent)		6.25%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Commitment, debt		$ 300,000	300,000
Notes payable and debt		$ 296,328	296,294
Debt interest rate (percent)		5.25%
Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Commitment, debt		$ 500,000	500,000
Notes payable and debt		492,723	492,521
Subordinated notes | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Commitment, debt		500,000	500,000
Notes payable and debt		$ 488,469	488,370
XL Cayman | Subordinated notes | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Debt interest rate (percent)	4.45%	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%	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]
Commitment, debt		$ 8,248	0
Notes payable and debt		$ 8,248	$ 0
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%