Fair Value Measurements - Unobservable Inputs Reconciliation - Assets (Details) - Fair Value, Measurements, Recurring - Level 3 - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	$ 74,377	$ 61,917
Purchases	203	14,943
Sales and cash collections	(12,330)	(10,869)
Realized gains	3,499	10,761
Unrealized gains	810	2,118
Unrealized losses	(16,043)	(1,752)
Balance at end of period	50,516	77,118
Investments in Associated Companies
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	52,240	40,643
Purchases	0	14,943
Sales and cash collections	(2,033)	0
Realized gains	0	0
Unrealized gains	810	2,118
Unrealized losses	(16,043)	(1,752)
Balance at end of period	34,974	55,952
Marketable Securities and Other
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at beginning of period	22,137	21,274
Purchases	203	0
Sales and cash collections	(10,297)	(10,869)
Realized gains	3,499	10,761
Unrealized gains	0	0
Unrealized losses	0	0
Balance at end of period	$ 15,542	$ 21,166