Significant Credit Concentrations (Details) - USD ($) $ in Billions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Maximum Credit Exposure by Category
On-balance sheet	$ 170	$ 147
Individuals [Member]
Maximum Credit Exposure by Category
On-balance sheet	112	98
Financial Institutions [Member]
Maximum Credit Exposure by Category
On-balance sheet	20	18
United States Government And Agencies [Member]
Maximum Credit Exposure by Category
On-balance sheet	3	3
Financial Services
Maximum Credit Exposure by Category
On-balance sheet	$ 35	$ 28

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=77997519&loc=d3e13537-108611

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