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Subsequent Events (Details) | 12 Months Ended
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Dec. 31, 2017 item
Subsequent Events [Abstract] |
Number of types of subsequent events | 2

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- Definition Number of types of subsequent events. + References No definition available. + Details
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Namespace Prefix: | sf_
Data Type: | xbrli:positiveIntegerItemType
Balance Type: | na
Period Type: | duration

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- References No definition available. + Details
Name: | us-gaap_SubsequentEventsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration