Reserves for unpaid losses and loss adjustment expenses Reconciliation of Claims Development to Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expense, net	$ 239,128
Reinsurance recoverable for unpaid losses	48,224	$ 70,199	$ 95,503
Unallocated claim adjustment expenses	28,365
Reserves for unpaid losses and loss adjustment expenses	315,717	$ 264,801	$ 219,629
Property [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expense, net	2,700
Reinsurance recoverable for unpaid losses	2,018
Casualty - claims made [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expense, net	44,142
Reinsurance recoverable for unpaid losses	7,092
Casualty - occurrence [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for claims and claim adjustment expense, net	192,286
Reinsurance recoverable for unpaid losses	$ 39,114