Investments (Troubled Debt Restructurings on Financing Receivables, Subsequent Defalts) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012 NumberOfLoans	Sep. 30, 2012 NumberOfLoans
Number of Loans, Subsequently Defaulted	0	1
Amount Defaulted	$ 0	$ 0.8
Retail [Member]
Number of Loans, Subsequently Defaulted	0	1
Amount Defaulted	$ 0	$ 0.8

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

No definition available.

Name:	sfg_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	sfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration