Transactions with related parties - Transactions with HPS (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2017
Consolidated statement of income (loss) items:
Investment management fees - related parties	$ 4,352	$ 4,409
Investment performance fees - related parties	0	5,800
Consolidated balance sheet items:
Other investments, at fair value	30,682		$ 30,461
Investment management and performance fees payable	$ 5,428		17,762
HPS
Related Party Transaction [Line Items]
Management fee percentage	1.00%
Performance fee percentage	10.00%
Additional performance fee percentage	25.00%
Minimum investment return for additional incentive fee	10.00%
Maximum incentive fee percentage	17.50%
Purchase of other investments			28,700	$ 50,000
Fair value of fund	$ 30,700
Company's ownership percentage of master fund	12.00%
Consolidated statement of income (loss) items:
Investment management fees - related parties	$ 4,099	4,147
Investment performance fees - related parties	0	5,800
Total related party expense	4,099	$ 9,947
Consolidated balance sheet items:
Other investments, at fair value	30,682		30,461
Investment management and performance fees payable	$ 5,428		$ 17,762