Composition of Certain Financial Statement Captions (Accrued Liabilities) (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Items Included in Consolidated Statement of Financial Condition [Abstract]
Accrued employee compensation	$ 75,745	$ 96,991
Accrued employee benefits and other personnel	13,870	9,222
Accrued consulting, outsourcing and professional fees	31,167	28,610
Accrued sub-advisory, distribution and other asset management fees	54,157	46,245
Accrued dividend payable	0	52,452
Other accrued liabilities	48,278	39,281
Total accrued liabilities	$ 223,217	$ 272,801