Contingent Liabilities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007	Sep. 30, 2012
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit outstanding		$ 4,300					$ 4,600
Standby letters of credit, unamortized fees		61.3					65.8
Minimum securities lending related collateral percentage		100.00%					100.00%
Possible loss in future periods in excess of accrual							40
Gain on redemption of Visa common stock					167.9
Remaining Visa shares held by Northern Trust, original cost basis					0
Visa indemnified litigation benefit (charges) recorded	10.1	23.1	33.0	17.8	76.1	(150.0)
Fair value of contingent consideration		56.8					48.9
Indemnification Agreement
Commitments and Contingencies Disclosure [Line Items]
Collateralized securities loaned, subject to indemnification		74,400					76,600
Credit loss liability		$ 0					$ 0