Investments (Amortized Costs And Fair Values Of Fixed Maturity Securities Available-For-Sale) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale, Amortized Cost	$ 7,431.5	$ 7,390.0
Total fixed maturity securities-available-for-sale, Unrealized Gains	473.7	423.1
Total fixed maturity securities-available-for-sale, Unrealized losses	(19.5)	(39.4)
Total fixed maturity securities-available-for-sale	7,885.7	7,773.7
Estimate of Fair Value, Fair Value Disclosure [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale	7,885.7	7,773.7
U.S. Government and Agency Bonds [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale, Amortized Cost	249.8	262.0
Total fixed maturity securities-available-for-sale, Unrealized Gains	39.6	38.3
Total fixed maturity securities-available-for-sale, Unrealized losses	0	(0.2)
U.S. Government and Agency Bonds [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale	289.4	300.1
U.S. State and Political Subdivision Bonds [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale, Amortized Cost	131.1	125.8
Total fixed maturity securities-available-for-sale, Unrealized Gains	11.2	10.4
Total fixed maturity securities-available-for-sale, Unrealized losses	(0.2)	(0.1)
U.S. State and Political Subdivision Bonds [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale	142.1	136.1
Foreign Government Bonds [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale, Amortized Cost	58.3	58.4
Total fixed maturity securities-available-for-sale, Unrealized Gains	6.7	6.5
Total fixed maturity securities-available-for-sale, Unrealized losses	0	0
Foreign Government Bonds [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale	65.0	64.9
Corporate Bonds [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale, Amortized Cost	6,992.3	6,943.8
Total fixed maturity securities-available-for-sale, Unrealized Gains	416.2	367.9
Total fixed maturity securities-available-for-sale, Unrealized losses	(19.3)	(39.1)
Corporate Bonds [Member] \| Estimate of Fair Value, Fair Value Disclosure [Member]
Schedule of Investments [Line Items]
Total fixed maturity securities-available-for-sale	$ 7,389.2	$ 7,272.6

Amount of accumulated unrealized gain before deducting unrealized loss on investments in available-for-sale securities impacting investments.

No definition available.

+ Details

Name:	sfg_AvailableforsaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Namespace Prefix:	sfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of accumulated unrealized loss before deducting unrealized gain on investments in available-for-sale securities impacting investments.

+ References

No definition available.

+ Details

Name:	sfg_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Namespace Prefix:	sfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of debt securities categorized neither as held-to-maturity nor trading.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (aa)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 320

-Section 50

-Paragraph 2

-Subparagraph (b)-(g)

-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesDebtSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_EstimateOfFairValueFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CorporateBondSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ForeignGovernmentDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: