Fair Value of Financial Assets and Liabilities - Fair Value Hierarchy of Financial Assets and Liabilities under FASB Guidance (Detail) (USD $)	Sep. 30, 2013	Dec. 31, 2012
Net amounts of recognized assets
Government debt	$ 32,029,000	$ 32,003,000
Marketable securities	36,935,000	0
Interest rate swaps	467,000	782,000
Foreign exchange swaps, Assets	134,000
Securities owned - Equities	124,000
Total	69,689,000	32,785,000
Net amounts of recognized Liabilities
Foreign exchange swaps, Liabilities	1,472,000	338,000
Total	1,472,000	338,000
Level 1 [Member]
Net amounts of recognized assets
Government debt	32,029,000	32,003,000
Marketable securities	36,935,000
Interest rate swaps
Foreign exchange swaps, Assets
Securities owned - Equities	124,000
Total	69,088,000	32,003,000
Net amounts of recognized Liabilities
Foreign exchange swaps, Liabilities
Total
Level 2 [Member]
Net amounts of recognized assets
Government debt
Marketable securities
Interest rate swaps	467,000	782,000
Foreign exchange swaps, Assets	134,000
Securities owned - Equities
Total	601,000	782,000
Net amounts of recognized Liabilities
Foreign exchange swaps, Liabilities	1,472,000	338,000
Total	1,472,000	338,000
Level 3 [Member]
Net amounts of recognized assets
Government debt
Marketable securities
Interest rate swaps
Foreign exchange swaps, Assets
Securities owned - Equities
Total
Net amounts of recognized Liabilities
Foreign exchange swaps, Liabilities
Total
Netting and Collateral [Member]
Net amounts of recognized assets
Government debt
Marketable securities
Interest rate swaps
Foreign exchange swaps, Assets
Securities owned - Equities
Total
Net amounts of recognized Liabilities
Foreign exchange swaps, Liabilities
Total