Borrowings (Narrative) (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
Debt Disclosure [Abstract]
FHLB borrowings repaid	$ 100.0
FHLB issued letters of credit	498.4
Long-term Debt and Capital Lease Obligations	$ 1.8

Amount of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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