Borrowings and Federal Home Loan Bank Advances (Narrative) (Details)		3 Months Ended	9 Months Ended
May 27, 2021 USD ($)	Sep. 30, 2021 USD ($) item	Sep. 30, 2021 USD ($) item
Short-term Debt [Line Items]
Uncommitted secured lines of credit			$ 880,000,000.0
Number of banks \| item		4	4
Daily borrowings under our uncommitted secured lines		$ 50,000,000.0	$ 50,000,000.0
Compensating balances		0	0
Federal Home Loan Bank Advances Floating Rate		52,000,000.0	52,000,000.0
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Credit facility expiration date	2026-05
Committed revolving credit facility with bank and broker dealer - subsidiary	$ 500,000,000.0
Credit facility sublimit	$ 200,000,000.0
Outstanding credit facility		$ 0	0
Company Owned Securities [Member]
Short-term Debt [Line Items]
Uncommitted secured lines of credit			$ 0
Federal Home Loan Bank advances [Member]
Short-term Debt [Line Items]
Weighted average interest rate on borrowings		0.28%	0.28%

Daily borrowings under credit arrangements.

No definition available.

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Namespace Prefix:	sf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line of credit facility expiration month and year.

+ References

No definition available.

+ Details

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Balance Type:	na
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- Definition

Line of Credit Facility Sublimit.

+ References

No definition available.

+ Details

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Namespace Prefix:	sf_
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Balance Type:	credit
Period Type:	instant

- Definition

Number of banks with which entity has committed revolving credit facility.

+ References

No definition available.

+ Details

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Namespace Prefix:	sf_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Uncommitted secured lines of credit.

+ References

No definition available.

+ Details

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Namespace Prefix:	sf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Weighted average interest rate on borrowings.

+ References

No definition available.

+ Details

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Period Type:	duration

- Definition

The amount of a specific compensating balance arrangement that is maintained under an agreement for a bank loan or future credit availability.

+ References

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

Par amount of advances made and reported by Federal Home Loan Bank (FHLBank) with floating interest rate.

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(22))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(16))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(16))
-URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910

+ Details

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Balance Type:	credit
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- Definition

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

+ References

+ Details

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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

No definition available.

+ Details

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- Details

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- Details

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- Details

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