Fair Value Measurements - Financial Instruments Not Measured at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Financial Assets:
Cash and cash equivalents	$ 850,190	$ 1,003,791	$ 1,209,695	$ 917,329
Deposits with banks	292,494	286,037
Cash deposited with clearing organizations and other segregated cash	65,232	75,506
Interest-bearing deposits (included within investments)	578	2,834
Financial Liabilities:
Deposits and other customer payables	269,763	288,427
Senior debt	1,076,850	1,076,850
Carrying Value [Member]
Financial Assets:
Cash and cash equivalents	850,190
Deposits with banks	292,494
Cash deposited with clearing organizations and other segregated cash	65,232
Interest-bearing financing receivables	76,481
Interest-bearing deposits (included within investments)	578
Financial Liabilities:
Deposits and other customer payables	269,763
Senior debt	1,076,850
Fair Value [Member]
Financial Assets:
Cash and cash equivalents	850,190
Deposits with banks	292,494
Cash deposited with clearing organizations and other segregated cash	65,232
Interest-bearing financing receivables	78,493
Interest-bearing deposits (included within investments)	578
Financial Liabilities:
Deposits and other customer payables	269,763
Senior debt	1,207,227
Fair Value [Member] | Level 1 [Member]
Financial Assets:
Cash and cash equivalents	850,190
Deposits with banks	292,494
Cash deposited with clearing organizations and other segregated cash	65,232
Interest-bearing deposits (included within investments)	578
Financial Liabilities:
Deposits and other customer payables	269,763
Fair Value [Member] | Level 2 [Member]
Financial Liabilities:
Senior debt	1,207,227
Fair Value [Member] | Level 3 [Member]
Financial Assets:
Interest-bearing financing receivables	$ 78,493