Commitments and contingencies (Lloyd's syndicate) (Details) - 9 months ended Sep. 30, 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 $1.2983	£1 equals $1.2983
Maximum premium levies assessable, amount	$ 23,369
2016 Underwriting year
Components Of Required Capital [Abstract]
Total	617,000
2015 Underwriting year
Components Of Required Capital [Abstract]
Total	$ 595,100