Commitments and contingencies (Lloyd's syndicate) (Details) - 12 months ended Dec. 31, 2016 - Lloyd's Syndicate 1183 £ in Thousands, $ in Thousands	USD ($)	GBP (£)
Funds at Lloyd's and Lloyd's Central Fund
Maximum premium levies assessable, percent	3.00%	3.00%
Estimated underwriting capacity | £		£ 600,000
Exchange rate	£1 equals $1.23	£1 equals $1.23
Maximum premium levies assessable, amount	$ 22,140
2017 Underwriting Year
Components Of Required Capital [Abstract]
Required capital	583,600
2016 underwriting year
Components Of Required Capital [Abstract]
Required capital	$ 617,000