Commitments and Contingencies (Other Commitments) (Details) - USD ($) $ in Millions	Jun. 30, 2025	Dec. 31, 2024
Broker-Dealer [Abstract]
Collateral securities	$ 642.2
Collateral security (Fair value)	899.1
Amount pledged with client-owned securities	507.6
Remaining collateral securities that can be re-pledged, loaned, or sold	391.5
Financial Instruments Owned and Pledged as Collateral [Line Items]
Remaining collateral securities that can be re-pledged, loaned, or sold	391.5
Options Clearing Corporation [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Marketable Securities	10.0	$ 8.5
National Securities Clearing Corporation [Member]
Financial Instruments Owned and Pledged as Collateral [Line Items]
Marketable Securities	$ 24.9	$ 20.0

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- Definition

The carrying value as of the balance sheet date of securities received as collateral from a counterparty that were re-pledged, sold, delivered or otherwise reused by the entity.

+ References

No definition available.

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- Definition

Fair value as of the balance sheet date of customer securities accepted as collateral by the entity that it is permitted by contract or custom to sell or re-pledge to counterparties as collateral.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481420/860-30-50-1A

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- Definition

Fair value as of the balance sheet date of customer securities accepted as collateral by the entity that it has sold or re-pledged to counterparties as collateral.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481420/860-30-50-1A

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- Definition

Fair value of collateral permitted, by contact or custom, to be sold or re-pledged.

+ References

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(11))
-Publisher FASB
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Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 860
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-Section 45
-Paragraph 1
-Publisher FASB
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- Definition

Amount of investment in marketable security.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(4))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

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