Contingent Liabilities and Guarantees - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2015 USD ($)
Commitments Contingencies And Guarantees [Line Items]
Weighted-average remaining term for outstanding commercial mortgage loans in years	7 years 10 months 6 days
Unpaid principal balance outstanding of loans sold	$ 6,100,000,000
Potential undiscounted future payments maximum limit	One-third of the principal balance of loans outstanding
Default guarantees, value	$ 188,000,000
Written Put Options [Member]
Commitments Contingencies And Guarantees [Line Items]
Default guarantees, value	$ 117,000,000
Weighted average life of written put options	2 years
Return Guarantee Agreement with LIHTC Investors [Member]
Commitments Contingencies And Guarantees [Line Items]
Compliance period of property	15 years
Default guarantees, value	$ 4,000,000
Expiration of guarantees	Extend through 2018
Standby Letters of Credit [Member]
Commitments Contingencies And Guarantees [Line Items]
Remaining weighted-average life of standby letters of credit in years	3 years
Remaining actual life letters of credit in years	From less than one year to as many as 11 years
Default guarantees, value	$ 63,000,000
Default Guarantees [Member]
Commitments Contingencies And Guarantees [Line Items]
Default guarantees, value	$ 0
Minimum [Member] | Low [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee Obligations	0.00%
Minimum [Member] | Moderate [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee Obligations	30.00%
Minimum [Member] | High [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee Obligations	70.00%
Maximum [Member] | Low [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee Obligations	30.00%
Maximum [Member] | Moderate [Member]
Commitments Contingencies And Guarantees [Line Items]
Guarantee Obligations	70.00%