Commercial Paper and Other Short-Term Borrowings - Narrative (Details) - USD ($)	1 Months Ended	12 Months Ended
Sep. 30, 2008	Dec. 31, 2015
Federal funds purchased and securities sold under repurchase agreements
Standby Letters of Credit
Weighted average remaining maturity days		4 days
Commercial paper
Standby Letters of Credit
Weighted average remaining maturity days		40 days
BTMU
Standby Letters of Credit
Revolving credit facility borrowing capacity	$ 500,000,000
Unsecured revolving credit facility years	3 years

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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

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