Consolidation - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Jun. 30, 2021	Dec. 31, 2020
Assets
Cash equivalents	$ 208,764,000	$ 207,101,000
Derivatives	0
Total assets measured at fair value	275,344,000	257,703,000
Liabilities
Derivatives	0
Level 1
Assets
Cash equivalents	208,764,000	207,101,000
Total assets measured at fair value	275,329,000	257,700,000
Level 2
Assets
Cash equivalents	0	0
Total assets measured at fair value	15,000	3,000
Level 3
Assets
Cash equivalents	0	0
Total assets measured at fair value	0	0
Fair Value, Measurements, Recurring | Consolidated Investment Products
Assets
Cash equivalents	121,934,000	82,295,000
Derivatives		2,085,000
Total assets measured at fair value	2,393,777,000	2,416,944,000
Liabilities
Notes payable	2,132,012,000	2,190,445,000
Derivatives		1,471,000
Short sales	531,000	520,000
Total liabilities measured at fair value	2,132,543,000	2,192,436,000
Fair Value, Measurements, Recurring | Debt investments | Consolidated Investment Products
Assets
Investments	2,245,970,000	2,289,426,000
Fair Value, Measurements, Recurring | Equity investments | Consolidated Investment Products
Assets
Investments	25,873,000	43,138,000
Fair Value, Measurements, Recurring | Level 1 | Consolidated Investment Products
Assets
Cash equivalents	121,934,000	82,295,000
Derivatives		858,000
Total assets measured at fair value	145,122,000	138,480,000
Liabilities
Notes payable	0	0
Derivatives		714,000
Short sales	531,000	520,000
Total liabilities measured at fair value	531,000	1,234,000
Fair Value, Measurements, Recurring | Level 1 | Debt investments | Consolidated Investment Products
Assets
Investments	1,281,000	16,859,000
Fair Value, Measurements, Recurring | Level 1 | Equity investments | Consolidated Investment Products
Assets
Investments	21,907,000	38,468,000
Fair Value, Measurements, Recurring | Level 2 | Consolidated Investment Products
Assets
Cash equivalents	0	0
Derivatives		1,227,000
Total assets measured at fair value	2,196,574,000	2,224,282,000
Liabilities
Notes payable	2,132,012,000	2,190,445,000
Derivatives		757,000
Short sales	0	0
Total liabilities measured at fair value	2,132,012,000	2,191,202,000
Fair Value, Measurements, Recurring | Level 2 | Debt investments | Consolidated Investment Products
Assets
Investments	2,194,503,000	2,219,199,000
Fair Value, Measurements, Recurring | Level 2 | Equity investments | Consolidated Investment Products
Assets
Investments	2,071,000	3,856,000
Fair Value, Measurements, Recurring | Level 3 | Consolidated Investment Products
Assets
Cash equivalents	0	0
Derivatives		0
Total assets measured at fair value	52,081,000	54,182,000
Liabilities
Notes payable	0	0
Derivatives		0
Short sales	0	0
Total liabilities measured at fair value	0	0
Fair Value, Measurements, Recurring | Level 3 | Debt investments | Consolidated Investment Products
Assets
Investments	50,186,000	53,368,000
Fair Value, Measurements, Recurring | Level 3 | Equity investments | Consolidated Investment Products
Assets
Investments	$ 1,895,000	$ 814,000