Acquisition and Disposals - Catlin - Overview (Details) £ / shares in Units, $ / shares in Units, £ in Millions, $ in Billions	May 01, 2015 USD ($) shares $ / shares	May 01, 2015 GBP (£) shares £ / shares	Sep. 30, 2017 $ / shares	Dec. 31, 2016 $ / shares
Business Acquisition [Line Items]
Common shares, par value (in dollars per share) | $ / shares			$ 0.01	$ 0.01
Catlin Group Limited
Business Acquisition [Line Items]
Total acquisition consideration | $	$ 4.1
Common 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	49,900,000	49,900,000
Cash consideration | £		£ 1,490