Goodwill and Intangible Assets - Finite-Lived Intangible Assets, Net of Accumulated Amortization (Parenthetical) (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Goodwill [Line Items]
Fixed income sales and trading business amount, excluded from intangible assets		$ 9.2
Weighted average useful life	11 years	11 years
Trading rights [Member]
Goodwill [Line Items]
Amortization period, in years	7 years	7 years
Other [Member]
Goodwill [Line Items]
Amortization period, in years	2 years	3 years
Customer and broker relationships [Member]
Goodwill [Line Items]
Amortization period, in years	14 years	13 years
Trade names [Member]
Goodwill [Line Items]
Amortization period, in years	13 years	13 years