Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Schedule of Equity Method Investments
Total revenues	$ 223,537	$ 202,702	$ 216,015	$ 52,488	$ 235,428	$ 173,166	$ 192,685	$ 217,909	$ 694,742	$ 819,188	$ 551,599
Net earnings	35,135	10,825	48,413	(2,062)	37,733	19,989	33,696	40,198	92,312	131,616	116,758
Change in fair value of derivative investments	$ 49,620	$ 31,520	$ 40,243	$ (106,629)	$ 60,080	$ 3,296	$ 17,828	$ 42,003
Class A
Schedule of Equity Method Investments
Net earnings									$ 89,701	$ 127,894	$ 113,456
Basic earnings per share (in dollars per share)	$ 9.94	$ 3.06	$ 13.69	$ (0.58)	$ 10.67	$ 5.65	$ 9.53	$ 11.37	$ 26.11	$ 37.22	$ 33.02
Diluted earnings per share (in dollars per share)	9.94	3.06	13.69	(0.58)	10.67	5.65	9.53	11.37	$ 26.11	$ 37.22	$ 33.02
Class B
Schedule of Equity Method Investments
Net earnings									$ 2,611	$ 3,722	$ 3,302
Basic earnings per share (in dollars per share)	4.97	1.53	6.85	(0.29)	5.34	2.83	4.76	5.68	$ 13.05	$ 18.61	$ 16.51
Diluted earnings per share (in dollars per share)	$ 4.97	$ 1.53	$ 6.85	$ (0.29)	$ 5.34	$ 2.83	$ 4.76	$ 5.68	$ 13.05	$ 18.61	$ 16.51