Commitments and Contingencies - Additional Information (Detail) (USD $)	12 Months Ended			1 Months Ended	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2013 Pending Litigation [Member]	Dec. 31, 2012 Auction Rate Securities [Member]
Loss Contingencies [Line Items]
Operating leases expiring at various dates through	2028
Rent expense	$ 51,200,000	$ 49,500,000	$ 50,400,000
Rent abatement credits	1,700,000
Capital commitments	5,100,000
Minimum estimated range of aggregate loss for legal proceedings	0
Maximum estimated range of aggregate loss for legal proceedings	125,500,000
Total amount of ARS the firm purchased and hold					77,100,000
Amount of ARS committed to purchase from clients					38.3
ARS in firm proprietary account					150,000
Contingency damages amount	30,000,000			30,000,000
Claim amount including interest and costs				140,000,000
Initial claim amount including interest and costs				$ 253,000,000