Investment information - Maturity profile of available for sale securities (Details) - Fixed maturities - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Due in one year or less - amortized cost	$ 15,055	$ 9,235
Due in one year or less - estimated fair value	$ 14,846	$ 9,248
Due in one year or less - % of fair value	2.10%	1.30%
Due after one to five years - amortized cost	$ 423,133	$ 414,235
Due after one to five years - estimated fair value	$ 428,376	$ 417,921
Due after one to five years - % of fair value	59.70%	56.00%
Due after five to ten years - amortized cost	$ 127,826	$ 133,822
Due after five to ten years - estimated fair value	$ 127,555	$ 136,329
Due after five to ten years - % of fair value	17.80%	18.30%
Due after ten years - amortized cost	$ 12,672	$ 11,833
Due after ten years - estimated fair value	$ 11,407	$ 12,026
Due after ten years - % of fair value	1.60%	1.60%
Total investments - available for sale - amortized cost	$ 749,835	$ 739,456
Total investments - available for sale - estimated fair value	$ 717,552	$ 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	$ 144,808	$ 145,555
No single maturity date - estimated fair value	$ 113,583	$ 145,434
No single maturity date - % of fair value	15.80%	19.50%
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
No single maturity date - amortized cost	$ 26,341	$ 24,776
No single maturity date - estimated fair value	$ 21,785	$ 24,750
No single maturity date - % of fair value	3.00%	3.30%