Fair Value Instruments - Financial Instruments Not Measured at Fair value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Financial Assets:
Cash and cash equivalents	$ 841,482	$ 850,190	$ 1,003,791	$ 1,209,695
Deposits with banks	244,879	292,494
Cash deposited with clearing organizations and other segregated cash	62,046	65,232
Interest-bearing deposits (included within investments)	516	578
Financial Liabilities:
Deposits and other customer payables	275,434	269,763
Senior debt	1,048,350	1,076,850
Carrying Value [Member]
Financial Assets:
Cash and cash equivalents	841,482	850,190
Deposits with banks	244,879	292,494
Cash deposited with clearing organizations and other segregated cash	62,046	65,232
Interest-bearing financing receivables	69,464	76,481
Interest-bearing deposits (included within investments)	516	578
Financial Liabilities:
Deposits and other customer payables	275,434	269,763
Senior debt	1,048,350	1,076,850
Fair Value [Member]
Financial Assets:
Cash and cash equivalents	841,482	850,190
Deposits with banks	244,879	292,494
Cash deposited with clearing organizations and other segregated cash	62,046	65,232
Interest-bearing financing receivables	71,433	78,493
Interest-bearing deposits (included within investments)	516	578
Financial Liabilities:
Deposits and other customer payables	275,434	269,763
Senior debt	1,117,247	1,207,227
Fair Value [Member] | Level 1 [Member]
Financial Assets:
Cash and cash equivalents	841,482	850,190
Deposits with banks	244,879	292,494
Cash deposited with clearing organizations and other segregated cash	62,046	65,232
Interest-bearing deposits (included within investments)	516	578
Financial Liabilities:
Deposits and other customer payables	275,434	269,763
Fair Value [Member] | Level 2 [Member]
Financial Liabilities:
Senior debt	1,117,247	1,207,227
Fair Value [Member] | Level 3 [Member]
Financial Assets:
Interest-bearing financing receivables	$ 71,433	$ 78,493