Acquisitions - Additional Information (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Aug. 27, 2012
Business Acquisition [Line Items]
Total consideration in shares			$ 0.08403
Total consideration to be paid	$ 4,400,000,000
Investment securities	5,660,831,000	6,074,361,000
Liabilities	72,389,000,000	72,806,210,000
Deposits	65,089,726,000	65,611,253,000
Long-term borrowing	5,394,563,000	4,607,758,000
Common stock [Member]
Business Acquisition [Line Items]
Percentage of total consideration			60.00%
Cash [Member]
Business Acquisition [Line Items]
Percentage of total consideration			40.00%
Hudson City [Member]
Business Acquisition [Line Items]
Assets	40,300,000,000
Loans	26,200,000,000
Investment securities	10,600,000,000
Liabilities	35,600,000,000
Deposits	23,200,000,000
Long-term borrowing	$ 12,000,000,000