Investment information - Maturity profile of available for sale securities (Details) - Fixed maturities - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Due in one year or less - amortized cost	$ 19,500	$ 9,235
Due in one year or less - estimated fair value	$ 19,525	$ 9,248
Due in one year or less - % of fair value	2.80%	1.30%
Due after one year through five years - amortized cost	$ 345,922	$ 414,235
Due after one year through five years - estimated fair value	$ 349,444	$ 417,921
Due after one year through five years - % of fair value	50.60%	56.00%
Due after five years through ten years - amortized cost	$ 152,830	$ 133,822
Due after five years through ten years - estimated fair value	$ 155,652	$ 136,329
Due after five years through ten years - % of fair value	22.60%	18.30%
Due after ten years - amortized cost	$ 14,387	$ 11,833
Due after ten years - estimated fair value	$ 13,259	$ 12,026
Due after ten years - % of fair value	1.90%	1.60%
Total investments, available for sale - amortized cost	$ 698,897	$ 739,456
Total investments, available for sale - estimated fair value	$ 690,225	$ 745,708
Total investments, available for sale - % of fair value	100.00%	100.00%
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
No single maturity date - amortized cost	$ 141,760	$ 145,555
No single maturity date - estimated fair value	$ 130,327	$ 145,434
No single maturity date - % of fair value	18.90%	19.50%
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
No single maturity date - amortized cost	$ 24,498	$ 24,776
No single maturity date - estimated fair value	$ 22,018	$ 24,750
No single maturity date - % of fair value	3.20%	3.30%