Investment Securities (Investment Income) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Available-for-sale Securities [Abstract]
Taxable interest	$ 336us-gaap_InterestIncomeSecuritiesTaxable	$ 297us-gaap_InterestIncomeSecuritiesTaxable	$ 262us-gaap_InterestIncomeSecuritiesTaxable
Taxable dividends	20us-gaap_DividendIncomeOperating	25us-gaap_DividendIncomeOperating	30us-gaap_DividendIncomeOperating
Interest and dividends exempt from U.S. federal income tax	11	3	0
Interest and dividends on available-for-sale investment securities	$ 367	$ 325	$ 292

Amount of operating dividend and interest income, including amortization and accretion of premiums and discounts, on available-for-sale securities.

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Available-for-Sale Securities

-URI http://asc.fasb.org/extlink&oid=6505594

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.2)

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 9

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 35

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=27723805&loc=d3e24721-111560

Name:	us-gaap_InterestAndDividendIncomeSecuritiesAvailableForSale
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration