Acquisition and Disposals - Acquisition narrative (Details) £ / shares in Units, $ / shares in Units, £ in Thousands, $ in Thousands				3 Months Ended	9 Months Ended
	Sep. 28, 2015 USD ($)	May. 01, 2015 USD ($) shares $ / shares	May. 01, 2015 GBP (£) shares £ / shares	Sep. 30, 2015 USD ($) $ / shares	Sep. 30, 2015 USD ($) $ / shares	Mar. 30, 2015 USD ($)	Jan. 08, 2015 GBP (£)	Dec. 31, 2014 USD ($) $ / shares
Business Acquisition [Line Items]
Ordinary shares, par value (in dollars per share) | $ / shares				$ 0.01	$ 0.01			$ 0.01
Goodwill				$ 1,203,594	$ 1,203,594			$ 415,936
Face amount of issued debt				2,763,217	2,763,217			$ 1,675,000
Goodwill acquired during the period					791,433
Indefinite-lived intangible assets acquired during the period					673,000
Finite-lived intangible assets acquired during the period					315,000
Intangibles Assets Arising from Insurance Contracts Acquired in Business Combination, Amortization Expense				140,400	$ 323,700
Catlin Group Limited
Business Acquisition [Line Items]
Ordinary shares, par value (in dollars per share) | $ / shares		$ 0.01
Cash price component, per share in GBP | £ / shares			£ 3.88
Exchange ratio per the merger agreement | shares		0.13	0.13
Share issuance to Catlin shareholders | shares		49,935,000	49,935,000
Cash consideration		$ 2,287,579	£ 1,490,377
Face amount of issued debt						$ 1,000,000
Goodwill acquired during the period		778,043
Indefinite-lived intangible assets acquired during the period		673,000
Finite-lived intangible assets acquired during the period		315,000
VOBA, amortization period					2 years
Adjustment to reserves, amortization period					20 years
Deferred tax liability		94,071
Acquisition related costs					$ 63,000
Business Combination Measurement Period					12 months
Catlin Group Limited | Operating expense
Business Acquisition [Line Items]
Acquisition related costs					$ 48,500
Catlin Group Limited | Interest expense
Business Acquisition [Line Items]
Acquisition related costs					14,500
Catlin Group Limited | Net deferred tax liability excluding valuation allowance
Business Acquisition [Line Items]
Deferred tax liability		22,100
Catlin Group Limited | Valuation allowance
Business Acquisition [Line Items]
Deferred tax liability		72,000
Catlin Group Limited | Intangible assets
Business Acquisition [Line Items]
Deferred tax liability		133,800
Catlin Group Limited | Loss carryforwards
Business Acquisition [Line Items]
Deferred tax liability		17,400
Catlin Group Limited | Unpaid losses and loss adjustment expenses, deferred acquisition costs and VOBA
Business Acquisition [Line Items]
Deferred tax liability		8,800
Catlin Group Limited | Other temporary differences
Business Acquisition [Line Items]
Deferred tax liability		500
Catlin Group Limited | Fixed assets
Business Acquisition [Line Items]
Deferred tax liability		13,000
Catlin Group Limited | Cash on hand
Business Acquisition [Line Items]
Cash consideration		1,310,000
Catlin Group Limited | Proceeds from debt issuance
Business Acquisition [Line Items]
Cash consideration		$ 980,600
Catlin Group Limited | XL Cayman
Business Acquisition [Line Items]
Available letter of credit facilities - commitments | £							£ 1,600,000
Catlin Inc
Business Acquisition [Line Items]
Release of valuation allowance, deferred tax asset	$ 47,500			$ 29,700	$ 17,800
Catlin Inc | XL Ireland
Business Acquisition [Line Items]
Percentage of voting interests acquired	100.00%