Regulatory Authorities - Additional Information (Detail) - USD ($)	Mar. 31, 2021	Dec. 31, 2020
LFNY [Member]
Regulatory Requirements [Line Items]
Specified fixed percentage, minimum required capital	6.67%
Minimum net capital requirement as defined under exchange act	$ 5,000
Regulatory capital	142,039,000
Regulatory capital in excess of minimum requirement	$ 138,286,000
Aggregate indebtedness to net capital ratio	0.40
LFNY [Member] | Maximum [Member]
Regulatory Requirements [Line Items]
Aggregate indebtedness to net capital ratio	15
U.K. Subsidiaries [Member]
Regulatory Requirements [Line Items]
Regulatory capital	$ 185,727,000
Regulatory capital in excess of minimum requirement	161,659,000
CFLF [Member]
Regulatory Requirements [Line Items]
Regulatory capital	141,077,000
Regulatory capital in excess of minimum requirement	72,867,000
Combined European Regulated Group [Member]
Regulatory Requirements [Line Items]
Regulatory capital		$ 184,842,000
Regulatory capital in excess of minimum requirement		$ 66,782,000
Other U.S. and Non-U.S. Subsidiaries [Member]
Regulatory Requirements [Line Items]
Regulatory capital	208,178,000
Regulatory capital in excess of minimum requirement	$ 179,051,000