Notes Payable and Debt and Financing Arrangements - Financing Structure (Details) - USD ($) $ in Thousands		Dec. 31, 2015	Mar. 30, 2015	Dec. 31, 2014	Sep. 15, 2014
Debt Instrument [Line Items]
Debt instrument, face amount	[1]	$ 2,675,000		$ 1,675,000
Notes payable and debt	[2]	2,644,970		1,662,580
Senior notes | 2.3% Senior Notes due 2018
Debt Instrument [Line Items]
Debt instrument, face amount	[1]	300,000		300,000
Notes payable and debt	[2]	$ 298,015		297,344
Debt instrument interest rate, stated percentage		2.30%
Senior notes | 5.75% Senior Notes due 2021
Debt Instrument [Line Items]
Debt instrument, face amount	[1]	$ 400,000		400,000
Notes payable and debt	[2]	$ 397,523		397,092
Debt instrument interest rate, stated percentage		5.75%
Senior notes | 6.375% Senior Notes due 2024
Debt Instrument [Line Items]
Debt instrument, face amount	[1]	$ 350,000		350,000
Notes payable and debt	[2]	$ 349,029		348,920
Debt instrument interest rate, stated percentage		6.375%
Senior notes | 6.25% Senior Notes due 2027
Debt Instrument [Line Items]
Debt instrument, face amount	[1]	$ 325,000		325,000
Notes payable and debt	[2]	$ 323,218		323,062
Debt instrument interest rate, stated percentage		6.25%
Senior notes | 5.25% Senior Notes due 2043
Debt Instrument [Line Items]
Debt instrument, face amount	[1]	$ 300,000		300,000
Notes payable and debt	[2]	$ 296,294		296,162
Debt instrument interest rate, stated percentage		5.25%
Subordinated debt | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Debt instrument, face amount	[1]		$ 500,000	0
Notes payable and debt	[2]	$ 492,521		0
Debt instrument interest rate, stated percentage		4.45%
Subordinated debt | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Debt instrument, face amount	[1]		$ 500,000	0
Notes payable and debt	[2]	$ 488,370		0
Debt instrument interest rate, stated percentage		5.50%
XL Cayman [Member] | Senior notes
Debt Instrument [Line Items]
Debt instrument interest rate, stated percentage					5.25%
XL Cayman [Member] | Subordinated debt | 4.45% Subordinated Notes due 2025
Debt Instrument [Line Items]
Debt instrument interest rate, stated percentage			4.45%
XL Cayman [Member] | Subordinated debt | 5.5% Subordinated Notes due 2045
Debt Instrument [Line Items]
Debt instrument interest rate, stated percentage			5.50%
Revolving credit facility
Debt Instrument [Line Items]
Available letter of credit facilities - commitments		$ 1,400,000		1,600,000
Available letter of credit facilities - in use		527,100		606,100
Line of credit facility, remaining borrowing capacity		822,900		968,900
Letter of credit facility
Debt Instrument [Line Items]
Available letter of credit facilities - commitments	[3],[4]	4,463,041		3,575,000
Available letter of credit facilities - in use	[4]	$ 2,515,653		$ 1,790,561

[1] Excluded from the table are certain credit facilities under which the Company is permitted to utilize up to $1.4 billion and $1.6 billion as of December 31, 2015 and December 31, 2014, respectively, for revolving loans to support general operating and financing needs. However, as of December 31, 2015 and December 31, 2014, $527.1 million and $606.1 million, respectively, were utilized under these facilities to issue letters of credit, leaving $822.9 million and $968.9 million, respectively, available to support other operating and financing need

[2] "In Use/Outstanding" data represent December 31, 2015 and December 31, 2014 accreted values.

[3] As of December 31, 2015 and December 31, 2014 the stated portion of allowable credit facilities permitted to be utilized for revolving loans was $1.4 billion and $1.6 billion, respectively. However, as of December 31, 2015 and December 31, 2014, $527.1 million and $606.1 million, respectively, of such facilities' limits were utilized to issue letters of credit, leaving $822.9 million and $968.9 million, respectively, available either to issue additional letters of credit or to support other operating or financing needs under these particular facilities.

[4] As of December 31, 2015 and December 31, 2014, there were fifteen and eight available credit facilities, respectively.