Deposits - Time Certificate Denominations and Overdrawn Accounts (Details) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015	Dec. 31, 2014
Deposits
Time certificates of deposit in denominations of $250,000 or more, in the aggregate	$ 2,800.0	$ 2,600.0	$ 2,300.0
Overdrawn deposit accounts classified as loans	$ 2.2	$ 3.0	$ 4.8

Amount of deposits held by the entity that have been reclassified as loan balances receivable, for example, but not limited to, overdrafts receivable.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=50488274&loc=d3e64164-112818

Name:	us-gaap_DepositLiabilitiesReclassifiedAsLoansReceivable1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant