Off-Balance Sheet Credit Risk (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
General settlement period of securities transactions	2 days
Fair value of securities accepted as collateral permitted to sell or repledge	$ 2,500.0	$ 2,400.0
Fair value of collateral securities sold or repledged	466.3	535.4
Outstanding commitments to originate loans	245.9	146.7
Letters of credit outstanding	26.2	26.3
Unused Lines Of Credit [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Unused lines of credit to commercial and consumer borrowers	$ 955.5	$ 919.5
Standby Letters of Credit [Member] \| Maximum [Member]
Fair Value Off Balance Sheet Risks Disclosure Information [Line Items]
Letters of credit, expiration period	1 year

General settlement period of securities transactions.

No definition available.

Name:	sf_GeneralSettlementPeriodOfSecuritiesTransactions
Namespace Prefix:	sf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Outstanding commitments to originate loans.

+ References

No definition available.

+ Details

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

- Definition

Unused lines of credit to commercial and consumer borrowers.

+ References

No definition available.

+ Details

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

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

Name:	us-gaap_CustomerSecuritiesForWhichEntityHasRightToSellOrRepledgeFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

Name:	us-gaap_CustomerSecuritiesForWhichEntityHasRightToSellOrRepledgeFairValueOfSecuritiesSoldOrRepledged
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

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

- Definition

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

+ References

No definition available.

+ Details

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Period remaining on line of credit facility before it terminates, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationPeriod
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: