Investment Securities - Realized Gains (Losses) on Investments (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	$ 7,573.5	$ 4,859.3
Debt Securities, Available-for-sale, Realized Gain	0.3	3.2	$ 0.0
Debt Securities, Available-for-sale, Realized Loss	(46.8)	(2.1)	$ 0.0
US Government Agencies Debt Securities [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	216.2	356.0
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	4,685.5	2,027.3
State, county and municipal securities
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	314.3	434.7
Mortgage-backed Securities, Issued by Private Enterprises [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	264.9	174.4
Corporate Debt Securities [Member]
Schedule of Available-for-sale Securities
Debt Securities, Available-for-sale, Amortized Cost	$ 272.3	$ 271.1

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27357-111563

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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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