Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	$ 89,872	$ 66,795
Trading	14,980	108,363
Held-to-maturity	49,807	49,748
Equity method investments	554	26
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	85,889	78,147
Equity investments at fair value	89,872	66,795
Trading	14,980	108,363
Total investments	155,213	224,932
Derivative - assets	644	1,225
Derivative - liabilities	556	2,386
Fair Value, Measurements, Recurring | Commodity futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative - assets	570	486
Derivative - liabilities	339	2,181
Fair Value, Measurements, Recurring | Commodity swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative - assets		739
Derivative - liabilities	173
Fair Value, Measurements, Recurring | Foreign exchange
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative - assets	74
Derivative - liabilities	44	205
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	85,889	78,147
Equity investments at fair value	89,292	62,942
Total investments	89,292	62,942
Derivative - assets	570	486
Derivative - liabilities	339	2,181
Fair Value, Measurements, Recurring | Level 1 | Commodity futures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative - assets	570	486
Derivative - liabilities	339	2,181
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	108	3,193
Trading	14,980	108,363
Total investments	15,088	111,556
Derivative - assets	74	739
Derivative - liabilities	217	205
Fair Value, Measurements, Recurring | Level 2 | Commodity swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative - assets		739
Derivative - liabilities	173
Fair Value, Measurements, Recurring | Level 2 | Foreign exchange
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative - assets	74
Derivative - liabilities	44	205
Fair Value, Measurements, Recurring | Investments Measured at NAV
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	472	660
Trading		0
Total investments	1,026	686
Company-sponsored funds | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	132	9,456
Company-sponsored funds | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	132	9,456
Common stocks | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	87,408	21,982
Common stocks | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	87,408	21,982
Fixed income | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading	14,980	108,363
Total investments	49,807	49,748
Fixed income | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading	14,980	108,363
Limited partnership interests | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	1,385	1,606
Limited partnership interests | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	1,048	1,056
Limited partnership interests | Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	0	605
Purchases / contributions	0	0
Sales / distributions	0	598
Realized gains	0	68
Unrealized gains (losses)	0	(61)
Transfers into (out of) level 3	0	0
Ending Balance	0	0
Limited partnership interests | Fair Value, Measurements, Recurring | Investments Measured at NAV
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	337	550
Preferred securities | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	812	33,641
Preferred securities | Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	704	30,448
Preferred securities | Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	108	3,193
Other | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	135	110
Other | Fair Value, Measurements, Recurring | Investments Measured at NAV
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity investments at fair value	135	110
Held-to-maturity Securities [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Held-to-maturity	49,807	49,748
Held-to-maturity Securities [Member] | Fixed income | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Held-to-maturity	49,807	49,748
Equity Method Investments [Member] | Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investments	554	26
Equity Method Investments [Member] | Fair Value, Measurements, Recurring | Investments Measured at NAV
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Equity method investments	$ 554	$ 26
The Cohen & Steers Global Realty Partners III-TE, L.P.
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Ownership percentage	0.20%
The Cohen & Steers Global Realty Focus Fund, a series of Cohen & Steers Series LP (GRF)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Ownership percentage	2.20%