Contingent Liabilities and Guarantees - Additional Information (Detail) (USD $)	9 Months Ended
	Sep. 30, 2012
Guarantor Obligations [Line Items]
Obligations under guarantees	$ 153,000,000
Return Guarantee Agreement with LIHTC Investors [Member]
Guarantor Obligations [Line Items]
Compliance period of property	15 years
Obligations under guarantees	43,000,000
Expiration of guarantees	Extend through 2018
Written Put Options [Member]
Guarantor Obligations [Line Items]
Obligations under guarantees	50,000,000
Weighted average life of written put options	1.8
Default Guarantees [Member]
Guarantor Obligations [Line Items]
Obligations under guarantees	1,000,000
Terms of default guarantees range	From less than one year to as many as seven years
Standby Letters of Credit [Member]
Guarantor Obligations [Line Items]
Remaining weighted-average life of standby letters of credit in years	2.9
Remaining actual life letters of credit in years	From less than one year to as many as eleven years
Obligations under guarantees	52,000,000
Recourse Agreement with FNMA [Member]
Guarantor Obligations [Line Items]
Weighted-average remaining term for outstanding commercial mortgage loans in yeras	6.4
Unpaid principal balance outstanding of loans sold	3,100,000,000
Potential undiscounted future payments maximum limit	One-third of the principal balance of loans outstanding
Obligations under guarantees	$ 7,000,000
Risk Level, Low | Minimum [Member]
Guarantor Obligations [Line Items]
Guarantee Obligations	0.00%
Risk Level, Low | Maximum [Member]
Guarantor Obligations [Line Items]
Guarantee Obligations	30.00%
Risk Level, Medium | Minimum [Member]
Guarantor Obligations [Line Items]
Guarantee Obligations	31.00%
Risk Level, Medium | Maximum [Member]
Guarantor Obligations [Line Items]
Guarantee Obligations	70.00%
Risk Level, High | Minimum [Member]
Guarantor Obligations [Line Items]
Guarantee Obligations	71.00%
Risk Level, High | Maximum [Member]
Guarantor Obligations [Line Items]
Guarantee Obligations	100.00%