Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2022	Dec. 31, 2021
Available-for-Sale:
Fair Value, Less than 12 Months	$ 5,650.9	$ 2,760.3
Gross Unrealized Losses, Less than 12 Months	(276.4)	(58.3)
Fair Value, 12 Months or Longer	149.2	68.7
Gross Unrealized Losses, 12 Months or Longer	(16.7)	(1.1)
Fair Value, Total	5,800.1	2,829.0
Gross Unrealized Losses, Total	(293.1)	(59.4)
US Treasury Notes Securities
Available-for-Sale:
Fair Value, Less than 12 Months	644.9
Gross Unrealized Losses, Less than 12 Months	(53.3)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	644.9
Gross Unrealized Losses, Total	(53.3)
U.S. Treasury notes
Available-for-Sale:
Fair Value, Less than 12 Months		684.7
Gross Unrealized Losses, Less than 12 Months		(12.9)
Fair Value, 12 Months or Longer		0.0
Gross Unrealized Losses, 12 Months or Longer		0.0
Fair Value, Total		684.7
Gross Unrealized Losses, Total		(12.9)
State, county, and municipal securities
Available-for-Sale:
Fair Value, Less than 12 Months	223.6	278.7
Gross Unrealized Losses, Less than 12 Months	(22.7)	(9.1)
Fair Value, 12 Months or Longer	29.6	5.0
Gross Unrealized Losses, 12 Months or Longer	(5.1)	(0.2)
Fair Value, Total	253.2	283.7
Gross Unrealized Losses, Total	(27.8)	(9.3)
Obligations of U.S. government agencies
Available-for-Sale:
Fair Value, Less than 12 Months	101.5	297.0
Gross Unrealized Losses, Less than 12 Months	(5.4)	(8.9)
Fair Value, 12 Months or Longer	15.4	16.4
Gross Unrealized Losses, 12 Months or Longer	(1.3)	(0.3)
Fair Value, Total	116.9	313.4
Gross Unrealized Losses, Total	(6.7)	(9.2)
U.S. agency residential & commercial mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Fair Value, Less than 12 Months	3,238.5	1,262.8
Gross Unrealized Losses, Less than 12 Months	(163.4)	(23.0)
Fair Value, 12 Months or Longer	84.9	26.4
Gross Unrealized Losses, 12 Months or Longer	(8.4)	(0.3)
Fair Value, Total	3,323.4	1,289.2
Gross Unrealized Losses, Total	(171.8)	(23.3)
Private mortgage-backed securities
Available-for-Sale:
Fair Value, Less than 12 Months	274.8	127.2
Gross Unrealized Losses, Less than 12 Months	(13.6)	(1.1)
Fair Value, 12 Months or Longer	0.0	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0	0.0
Fair Value, Total	274.8	127.2
Gross Unrealized Losses, Total	(13.6)	(1.1)
Collateralized Loan Obligations
Available-for-Sale:
Fair Value, Less than 12 Months	1,020.9
Gross Unrealized Losses, Less than 12 Months	(4.5)
Fair Value, 12 Months or Longer	0.0
Gross Unrealized Losses, 12 Months or Longer	0.0
Fair Value, Total	1,020.9
Gross Unrealized Losses, Total	(4.5)
Corporate Debt Securities
Available-for-Sale:
Fair Value, Less than 12 Months	146.7	109.9
Gross Unrealized Losses, Less than 12 Months	(13.5)	(3.3)
Fair Value, 12 Months or Longer	19.3	20.9
Gross Unrealized Losses, 12 Months or Longer	(1.9)	(0.3)
Fair Value, Total	166.0	130.8
Gross Unrealized Losses, Total	$ (15.4)	$ (3.6)

No definition available.

Name:	us-gaap_AvailableForSaleSecuritiesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

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.

+ References

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=124269663&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=123581744&loc=d3e27337-111563

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on 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 12 months or longer, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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 less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: 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=123581744&loc=d3e27337-111563

Reference 2: 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=124269663&loc=SL82922890-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on 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 less than 12 months, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position without allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=123581744&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=124269663&loc=SL82922888-210455

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=121590138&loc=SL82922954-210456

+ Details

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

- Definition

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

+ References

+ Details

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

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasuryNotesSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USTreasurySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByUSGovernmentSponsoredEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesIssuedByPrivateEnterprisesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedLoanObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: