Investment Securities (Schedule of AFS and HTM - Estimated Unrealized Losses) (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Debt securities, Fair Value	$ 436.9	$ 2,078.1
Total securities available-for-sale, Less than 12 months, Debt securities, Gross Unrealized Loss	(2.2)	(11.7)
Total securities available-for-sale, 12 months or greater, Debt securities, Fair Value	37.2	878.9
Total securities available-for-sale, 12 months or greater, Debt securities, Gross Unrealized Loss	(0.8)	(12.2)
U.S. government/sponsored agency – Residential
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Debt securities, Fair Value	195.8	1,396.5
Total securities available-for-sale, Less than 12 months, Debt securities, Gross Unrealized Loss	(1.4)	(4.6)
Total securities available-for-sale, 12 months or greater, Debt securities, Fair Value	37.2	861.3
Total securities available-for-sale, 12 months or greater, Debt securities, Gross Unrealized Loss	(0.8)	(12.1)
U.S. government/sponsored agency – Commercial
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Debt securities, Fair Value	175.4	282.7
Total securities available-for-sale, Less than 12 months, Debt securities, Gross Unrealized Loss	(0.6)	(1.7)
Total securities available-for-sale, 12 months or greater, Debt securities, Fair Value	0.0	17.6
Total securities available-for-sale, 12 months or greater, Debt securities, Gross Unrealized Loss	0.0	(0.1)
U.S. government/sponsored agency obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Debt securities, Fair Value		398.9
Total securities available-for-sale, Less than 12 months, Debt securities, Gross Unrealized Loss		(5.4)
Total securities available-for-sale, 12 months or greater, Debt securities, Fair Value		0.0
Total securities available-for-sale, 12 months or greater, Debt securities, Gross Unrealized Loss		$ 0.0
Corporate bonds - foreign
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale, Less than 12 months, Debt securities, Fair Value	65.7
Total securities available-for-sale, Less than 12 months, Debt securities, Gross Unrealized Loss	(0.2)
Total securities available-for-sale, 12 months or greater, Debt securities, Fair Value	0.0
Total securities available-for-sale, 12 months or greater, Debt securities, Gross Unrealized Loss	$ 0.0

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922890-210455

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
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