Investment Securities - Fair value of investment securities (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Fair value of investments
Cash equivalents	$ 196,931	$ 156,849
Available for sale securities	250,764	265,198
Trading debt securities	118,222	124,390
Equity securities	200,309	215,716
Total investment securities	617,135	700,492
Amount of transfer between levels	0	0
Money market funds
Fair value of investments
Cash equivalents	121,759	145,785
U.S. government sponsored enterprise note
Fair value of investments
Cash equivalents	895
Certificates of deposit
Fair value of investments
Available for sale securities	5,001	12,999
Trading debt securities		1,999
Commercial paper
Fair value of investments
Cash equivalents	74,277	11,064
Available for sale securities	7,970	34,978
Trading debt securities	1,993
Corporate bonds
Fair value of investments
Available for sale securities	218,121	197,442
Trading debt securities	77,250	55,414
U.S. treasury bills
Fair value of investments
Available for sale securities	19,672	19,779
Trading debt securities	5,884	4,929
Fixed income securities: Mortgage-backed securities
Fair value of investments
Trading debt securities	7	10
Common Stock
Fair value of investments
Equity securities	21,204	116
Consolidated Sponsored Funds
Fair value of investments
Trading debt securities	33,088	62,038
Equity securities	24,879	77,048
Sponsored funds
Fair value of investments
Equity securities	153,548	137,857
Equity method securities	47,840	95,188
Sponsored privately offered funds
Fair value of investments
Equity securities	678	695
Level 1
Fair value of investments
Cash equivalents	121,759	145,785
Total investment securities	247,459	310,209
Level 1 \| Money market funds
Fair value of investments
Cash equivalents	121,759	145,785
Level 1 \| Common Stock
Fair value of investments
Equity securities	21,192	116
Level 1 \| Consolidated Sponsored Funds
Fair value of investments
Equity securities	24,879	77,048
Level 1 \| Sponsored funds
Fair value of investments
Equity securities	153,548	137,857
Equity method securities	47,840	95,188
Level 2
Fair value of investments
Cash equivalents	75,172	11,064
Total investment securities	368,986	389,588
Level 2 \| U.S. government sponsored enterprise note
Fair value of investments
Cash equivalents	895
Level 2 \| Certificates of deposit
Fair value of investments
Available for sale securities	5,001	12,999
Trading debt securities		1,999
Level 2 \| Commercial paper
Fair value of investments
Cash equivalents	74,277	11,064
Available for sale securities	7,970	34,978
Trading debt securities	1,993
Level 2 \| Corporate bonds
Fair value of investments
Available for sale securities	218,121	197,442
Trading debt securities	77,250	55,414
Level 2 \| U.S. treasury bills
Fair value of investments
Available for sale securities	19,672	19,779
Trading debt securities	5,884	4,929
Level 2 \| Fixed income securities: Mortgage-backed securities
Fair value of investments
Trading debt securities	7	10
Level 2 \| Consolidated Sponsored Funds
Fair value of investments
Trading debt securities	33,088	62,038
Level 3
Fair value of investments
Total investment securities	12
Level 3 \| Common Stock
Fair value of investments
Equity securities	12
Other Assets Held at Net Value
Fair value of investments
Total investment securities	678	695
Other Assets Held at Net Value \| Sponsored privately offered funds
Fair value of investments
Equity securities	$ 678	$ 695

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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