Summary of Significant Accounting Policies (Nonmarketable Equity Securities) (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
FHLB stock minimum redemption period	5 years
FRB stock minimum redemption period	6 months
FRB stock minimum annual amount redeemable	25.00%
Corporation's ownership in FHLB stock	$ 112.0	$ 58.0
Corporation's ownership in FRB stock	$ 68.1	$ 39.4

Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 325
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75031641&loc=d3e63345-112809

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