Significant Credit Concentrations (Details) (USD $) In Billions, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Maximum Credit Exposure by Category
On-balance sheet	$ 141	$ 142
Individuals [Member]
Maximum Credit Exposure by Category
On-balance sheet	95	92
Unused lines-of-credit	253	238
Financial Institutions [Member]
Maximum Credit Exposure by Category
On-balance sheet	25	28
United States Government And Agencies [Member]
Maximum Credit Exposure by Category
On-balance sheet	5	6
Other Concentration [Member]
Maximum Credit Exposure by Category
On-balance sheet	$ 16	$ 16

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 825

-SubTopic 10

-Section 50

-Paragraph 21

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7491637&loc=d3e13537-108611

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 107

-Paragraph 15A

-Subparagraph b

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration