Federal Home Loan Bank Advances (Narrative) (Detail) (USD $)	Dec. 31, 2012	Dec. 31, 2011
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Loans pledged as collateral to the Federal Home Loan Bank	$ 3,700,000,000	$ 3,900,000,000
Unused borrowing capacity in Line of Credit	5,000,000	5,000,000
Securities Pledged as Collateral [Member]
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Additional borrowing capacity	$ 500,000,000	$ 1,000,000,000

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant