Regulatory Authorities - Additional Information (Detail) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
LFNY [Member]
Regulatory Requirements [Line Items]
Specified fixed percentage, minimum required capital	6.67%
Minimum net capital requirement as defined under exchange act	$ 100,000
Regulatory capital	65,442,000
Regulatory capital in excess of minimum requirement	$ 62,369,000
Aggregate indebtedness to net capital ratio	0.70
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	$ 196,268,000
Regulatory capital in excess of minimum requirement	177,118,000
CFLF [Member]
Regulatory Requirements [Line Items]
Regulatory capital	144,257,000
Regulatory capital in excess of minimum requirement	84,537,000
Combined European Regulated Group [Member]
Regulatory Requirements [Line Items]
Regulatory capital		$ 170,933,000
Regulatory capital in excess of minimum requirement		$ 68,025,000
Other U.S. and Non-U.S. Subsidiaries [Member]
Regulatory Requirements [Line Items]
Regulatory capital	140,069,000
Regulatory capital in excess of minimum requirement	$ 110,736,000