Reserves for unpaid losses and loss adjustment expenses Liability for Unpaid Losses and Loss Adjustment Expenses (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense	$ (7,037)	$ (11,280)	$ (12,714)
Accident Year 2017 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense	(6,800)
Accident Year 2016 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense	(3,800)	(9,400)
Accident Year 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense		(4,200)	(6,100)
Accident Years 2011 to 2015 [Member] [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense	$ 3,600
Accident Year 2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense		(1,800)
Accident Year 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense		2,100
Accident Year 2012 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense		1,300
Accident Year 2011 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense		$ 700
Accident year 2014 and prior [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense			$ (6,600)