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, 2019	Dec. 31, 2018	Dec. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense	$ (9,423)	$ (7,037)	$ (11,280)
Commutation of 2015 MLQS	0	0	27,929
Accident Year 2018 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense	(13,000)
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	(1,600)	(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)
us-gaap_ShortdurationInsuranceContractsAccidentYear2015and2014 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense			(6,000)
Accident Years 2011 to 2015 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense	$ 5,200	$ 3,600
Accident Years 2011 to 2013 [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Prior year losses and loss adjustment expense			$ 4,100