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Fair Value Of Financial Instruments (Narrative) (Details) $ in Millions	12 Months Ended
	Dec. 31, 2018 USD ($)
Fair Value Disclosures [Abstract]
Transfers of financial assets from Level 2 to Level 1	$ 0.9
Transfers of financial assets from Level 1 to Level 2	0.8
Transfers of financial assets into Level 3	$ 0.0

X

- Definition

Fair value level 1 to level 2 transfers amount

+ References

No definition available.

+ Details

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Namespace Prefix:	sf_
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Balance Type:	debit
Period Type:	duration

X

- Definition

Fair Value Level 2 To Level 1 Transfers Amount.

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
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Period Type:	duration

X

- Definition

Amount of transfer of financial instrument classified as an asset into level 3 of the fair value hierarchy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

+ Details

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