Fair Value Measurements - Unobservable Inputs Reconciliation - Assets (Details) - Fair Value, Measurements, Recurring - Level 3 - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2020	Jun. 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	204	14,943
Sales and cash collections	(8,880)	(7,557)
Realized gains	2,660	6,677
Unrealized gains	810	883
Unrealized losses	(18,329)
Balance at end of period	50,842	76,863
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	883
Unrealized losses	(18,329)
Balance at end of period	32,688	56,469
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	204	0
Sales and cash collections	(6,847)	(7,557)
Realized gains	2,660	6,677
Unrealized gains	0	0
Unrealized losses	0
Balance at end of period	$ 18,154	$ 20,394