Acquisition and Disposals - Acquisition narrative (Details) £ / shares in Units, $ / shares in Units, £ in Thousands, $ in Thousands			6 Months Ended
	May. 01, 2015 USD ($) shares $ / shares	May. 01, 2015 GBP (£) shares £ / shares	Jun. 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
Goodwill			$ 1,190,230			$ 415,936
Face amount of issued debt			2,763,142			$ 1,675,000
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
Intangibles Assets Arising from Insurance Contracts Acquired in Business Combination, Amortization Expense			$ 183,300
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			$ 61,800
Business Combination Measurement Period			12 months
Catlin Group Limited | Operating expense
Business Acquisition [Line Items]
Acquisition related costs			$ 47,300
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