Related party transactions (Details) - USD ($)				12 Months Ended
	Nov. 07, 2014	Oct. 02, 2014	Dec. 20, 2011	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Group Ark Insurance Holdings Ltd
Related Party Transaction [Line Items]
Gross premiums written				$ 3,157,000	$ 2,718,000	$ 2,073,000
Premiums receivable				292,000	82,000
Reinsurance premiums ceded				40,000	24,000	126,000
Related Party Transaction Reinsurance Balances Payable				0	4,000
Related Party Transaction Loss Reserves Recoverable				798,000	790,000
Earned premium adjustments				3,115,000	2,833,000	1,982,000
Wellington Insurance Company [Member]
Related Party Transaction [Line Items]
Premiums receivable				666,000	0
Reinsurance premiums ceded						0
Related Party Transaction Reinsurance Balances Payable				2,860,000	0	0
Earned premium adjustments				$ 2,603,000	0
Conning Inc
Related Party Transaction [Line Items]
Investment management fees					841,000	1,125,000
Aquiline Capital Partners II GP Offshore Ltd
Related Party Transaction [Line Items]
Percentage of interest assumed				100.00%
Capital commitment				$ 50,000,000
Partnership fees incurred				843,000	1,519,000	410,000
Payments for (Proceeds from) Limited Partnership				$ (9,032,000)	6,093,000	12,468,000
Aquiline Financial Services Fund II LP
Related Party Transaction [Line Items]
Percentage of interest assumed				100.00%
Capital commitment		$ 10,000,000	$ 50,000,000
Aquiline Financial Services Fund III LP
Related Party Transaction [Line Items]
Capital commitment	$ 100,000,000
Partnership fees incurred				$ 2,031,000	1,814,000
Payments for (Proceeds from) Limited Partnership				23,873,000	$ 13,793,000	$ 0
Western World Insurance Group, Inc | Aquiline Capital Partners II GP Offshore Ltd
Related Party Transaction [Line Items]
Capital commitment		$ 10,000,000		$ 10,000,000