Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	$ 283.3	$ 925.4
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(6.7)	(5.4)
Available-for-Sale, 12 Months or More, Fair Value	213.5	202.5
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(2.8)	(1.8)
Available-for-Sale, Total Fair Value	496.8	1,127.9
Available-for-Sale, Total Gross Unrealized Losses	(9.5)	(7.2)
Obligations of U.S. government agencies
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	19.9	185.3
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(0.1)	(0.8)
Available-for-Sale, 12 Months or More, Fair Value	0.0	0.0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	0.0	0.0
Available-for-Sale, Total Fair Value	19.9	185.3
Available-for-Sale, Total Gross Unrealized Losses	(0.1)	(0.8)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	143.5	740.1
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(1.7)	(4.6)
Available-for-Sale, 12 Months or More, Fair Value	133.6	155.9
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(1.0)	(1.4)
Available-for-Sale, Total Fair Value	277.1	896.0
Available-for-Sale, Total Gross Unrealized Losses	(2.7)	(6.0)
Private mortgage-backed securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	0.0	0.0
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	0.0	0.0
Available-for-Sale, 12 Months or More, Fair Value	40.0	46.6
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(1.2)	(0.4)
Available-for-Sale, Total Fair Value	40.0	46.6
Available-for-Sale, Total Gross Unrealized Losses	(1.2)	$ (0.4)
Corporate securities
Available-for-Sale:
Available-for-Sale, Less than 12 Months, Fair Value	119.9
Available-for-sale, Less than 12 Months, Gross Unrealized Losses	(4.9)
Available-for-Sale, 12 Months or More, Fair Value	39.9
Available-for-Sale, 12 Months or More, Gross Unrealized Losses	(0.6)
Available-for-Sale, Total Fair Value	159.8
Available-for-Sale, Total Gross Unrealized Losses	$ (5.5)

No definition available.

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

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 7
-URI http://asc.fasb.org/extlink&oid=121645371&loc=d3e27337-111563

+ Details

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

- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

No definition available.

+ Details

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

- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for less than twelve months.

+ References

No definition available.

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

+ References

No definition available.

+ Details

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

- Definition

+ References

No definition available.

+ Details

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

- 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_CorporateDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: