Transactions with related parties - Transactions with HPS (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Dec. 31, 2017
Consolidated statement of income (loss) items:
Investment management fees - related parties	$ 4,262	$ 4,570	$ 8,614	$ 8,979
Investment performance fees - related parties	0	1,692	0	7,492
Consolidated balance sheet items:
Other investments, at fair value	34,142		34,142		$ 30,461
Investment management and performance fees payable	$ 5,511		$ 5,511		17,762
HPS
Related Party Transaction [Line Items]
Management fee percentage	1.00%		1.00%
Management fee AUM threshold	$ 1,500,000		$ 1,500,000
Management fee, percentage that applies above threshold	0.75%		0.75%
Performance fee percentage	10.00%		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	$ 34,100		$ 34,100
Company's ownership percentage of master fund	12.00%		12.00%
Consolidated statement of income (loss) items:
Investment management fees - related parties	$ 4,039	4,297	$ 8,138	8,444
Investment performance fees - related parties	0	1,692	0	7,492
Total related party expense	4,039	$ 5,989	8,138	$ 15,936
Consolidated balance sheet items:
Other investments, at fair value	34,142		34,142		30,461
Investment management and performance fees payable	$ 5,511		$ 5,511		$ 17,762