Commitments and contingencies (Lloyd's syndicate) (Details) - Lloyd's Syndicate 1183 £ in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2018 GBP (£)	Dec. 31, 2017 USD ($)
Funds at Lloyd's and Lloyd's Central Fund
Maximum premium levies assessable, percent	3.00%	3.00%
Estimated underwriting capacity | £		£ 650,000
Exchange rate	£1 equals $1.40	£1 equals $1.40
Maximum premium levies assessable, amount	$ 27,300		$ 26,325
2018 Underwriting Year
Loss Contingencies [Line Items]
Components of required capital	$ 661,600
2017 Underwriting Year
Loss Contingencies [Line Items]
Components of required capital			$ 583,600