Marketable Securities (Details) - USD ($)		3 Months Ended		12 Months Ended
	Jun. 28, 2013	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Nov. 30, 2019
Marketable Securities [Line Items]
Gross proceeds from sale of marketable securities		$ 34,591,000	$ 9,106,000
Gain (loss) on marketable securities		2,204,000	51,000
Unrealized (gains) loss on marketable securities		0	$ 3,960,000
Marketable securities		$ 0		$ 36,795,000	$ 98,600,000
Nasdaq Omx
Marketable Securities [Line Items]
Amount recognized in connection with the earn-out including other income (loss) (in shares)				992,247
Number of shares sold in transaction (in shares)		343,562	100,000
Nasdaq Omx | BGC Partners Inc
Marketable Securities [Line Items]
Period for earn-out receivable under common stock transaction	15 years	8 years
Maximum | Nasdaq Omx
Marketable Securities [Line Items]
Remaining earn-out receivable under common stock transaction (in shares)		8,930,223
Maximum | Nasdaq Omx | BGC Partners Inc
Marketable Securities [Line Items]
Earn-out shares receivable under common stock transaction (in shares)	14,883,705
Minimum | Nasdaq Omx
Marketable Securities [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000.0
Minimum | Nasdaq Omx | BGC Partners Inc
Marketable Securities [Line Items]
Gross revenue on earn-out receivable per year under common stock transaction	$ 25,000,000.0