Investment Securities - Narrative (Details) $ in Billions	3 Months Ended
Mar. 31, 2018 USD ($)
Hedge accounting
Debt Securities, Available-for-sale [Line Items]
HTM securities transferred to AFS, carrying value	$ 22.4

Amount of investment in debt security, measured at amortized cost, transferred to investment in debt security, measured at fair value, with change in fair value recognized in other comprehensive income (available-for-sale) and net income (trading).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 10
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27405-111563

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