Investment Securities (Carrying Value And Fair Value Of Securities Held-To-Maturity) (Details) $ in Millions	Dec. 31, 2017 USD ($)
Mortgage-backed Securities, U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	$ 110.0
Gross Unrealized Gains	0.7
Gross Unrealized Losses	(3.3)
Fair Value	107.4
State and municipal
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	27.7
Gross Unrealized Gains	0.0
Gross Unrealized Losses	(0.5)
Fair Value	27.2
Foreign government
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	2.4
Gross Unrealized Gains	0.0
Gross Unrealized Losses	0.0
Fair Value	2.4
Corporate — foreign
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	102.9
Gross Unrealized Gains	6.2
Gross Unrealized Losses	0.0
Fair Value	109.1
Debt securities
Schedule of Held-to-maturity Securities [Line Items]
Carrying Value	243.0
Gross Unrealized Gains	6.9
Gross Unrealized Losses	(3.8)
Fair Value	$ 246.1

Amount after other than temporary impairment (OTTI) accretion, of investments in debt securities classified as held-to-maturity.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (f)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.6)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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