Indemnifications and Guarantees - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013 Transactions Financial_Institution	Dec. 31, 2012 Transactions
Guarantor Obligations [Line Items]
Number of unrelated financial institutions	3
Derivative transaction held for guarantee	97	95
Aggregate notional amount	$ 367.6	$ 381.7
Notional amount maturity period	5 months
Notional amount maturity period	12 years 6 months
Aggregate market value of swaps to customers	$ 31.5	$ 35.5
Minimum [Member]
Guarantor Obligations [Line Items]
Term of loan	15 years
Maximum [Member]
Guarantor Obligations [Line Items]
Term of loan	30 years