Retirement Benefits (Reconciliation Of Changes In Pension Plans' Projected Benefit Obligations, Fair Value Of Plan Assets And Funded Status) (Details) (Defined Benefit Pension [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Pension [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of the year	$ (408.4)	$ (418.5)	$ (371.6)
Service cost	(8.4)	(10.2)	(9.7)
Interest cost, net periodic benefit	(19.3)	(16.7)	(16.5)
Actuarial loss	(87.1)	26.0	(29.7)
Benefits Paid	12.2	11.0	9.0
Benefit obligation at end of the year	(511.0)	(408.4)	(418.5)
Fair value of plan assets at beginning of the year	440.4	386.0	349.7
Actual return on plan assets	28.8	65.4	35.4
Employer contributions	15.0	0	10.0
Benefits paid and estimated expenses	(12.3)	(11.0)	(9.1)
Fair value of plan assets at end of the year	471.9	440.4	386.0
Funded status at end of the year	$ (39.1)	$ 32.0	$ (32.5)

Retirement Benefits (Reconciliation Of Changes In Pension Plans' Projected Benefit Obligations, Fair Value Of Plan Assets And Funded Status) (Details) (Defined Benefit Pension [Member], USD $)
In Millions, unless otherwise specified

12 Months Ended

Dec. 31, 2014

Dec. 31, 2013

Dec. 31, 2012

Defined Benefit Pension [Member]

Defined Benefit Plan Disclosure [Line Items]

Benefit obligation at beginning of the year

$ (408.4)us-gaap_DefinedBenefitPlanBenefitObligation
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$ (418.5)us-gaap_DefinedBenefitPlanBenefitObligation
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= us-gaap_PensionPlansDefinedBenefitMember

$ (371.6)us-gaap_DefinedBenefitPlanBenefitObligation
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= us-gaap_PensionPlansDefinedBenefitMember

Service cost

(8.4)us-gaap_DefinedBenefitPlanServiceCost
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(10.2)us-gaap_DefinedBenefitPlanServiceCost
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= us-gaap_PensionPlansDefinedBenefitMember

(9.7)us-gaap_DefinedBenefitPlanServiceCost
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Interest cost, net periodic benefit

(19.3)sfg_InterestCostPeriodicBenefitCost
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(16.7)sfg_InterestCostPeriodicBenefitCost
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(16.5)sfg_InterestCostPeriodicBenefitCost
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Actuarial loss

(87.1)us-gaap_DefinedBenefitPlanActuarialGainLoss
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26.0us-gaap_DefinedBenefitPlanActuarialGainLoss
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= us-gaap_PensionPlansDefinedBenefitMember

(29.7)us-gaap_DefinedBenefitPlanActuarialGainLoss
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Benefits Paid

12.2us-gaap_DefinedBenefitPlanBenefitsPaid
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11.0us-gaap_DefinedBenefitPlanBenefitsPaid
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9.0us-gaap_DefinedBenefitPlanBenefitsPaid
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= us-gaap_PensionPlansDefinedBenefitMember

Benefit obligation at end of the year

(511.0)us-gaap_DefinedBenefitPlanBenefitObligation
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(408.4)us-gaap_DefinedBenefitPlanBenefitObligation
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
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(418.5)us-gaap_DefinedBenefitPlanBenefitObligation
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Fair value of plan assets at beginning of the year

440.4us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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386.0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_PensionPlansDefinedBenefitMember

349.7us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_PensionPlansDefinedBenefitMember

Actual return on plan assets

28.8us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
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65.4us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
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35.4us-gaap_DefinedBenefitPlanActualReturnOnPlanAssets
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Employer contributions

15.0us-gaap_DefinedBenefitPlanContributionsByEmployer
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= us-gaap_PensionPlansDefinedBenefitMember

0us-gaap_DefinedBenefitPlanContributionsByEmployer
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10.0us-gaap_DefinedBenefitPlanContributionsByEmployer
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember

Benefits paid and estimated expenses

(12.3)sfg_BenefitsPaidAndEstimatedExpenses
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(11.0)sfg_BenefitsPaidAndEstimatedExpenses
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(9.1)sfg_BenefitsPaidAndEstimatedExpenses
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= us-gaap_PensionPlansDefinedBenefitMember

Fair value of plan assets at end of the year

471.9us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_PensionPlansDefinedBenefitMember

440.4us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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386.0us-gaap_DefinedBenefitPlanFairValueOfPlanAssets
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= us-gaap_PensionPlansDefinedBenefitMember

Funded status at end of the year

$ (39.1)us-gaap_DefinedBenefitPlanFundedStatusOfPlan
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$ 32.0us-gaap_DefinedBenefitPlanFundedStatusOfPlan
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$ (32.5)us-gaap_DefinedBenefitPlanFundedStatusOfPlan
/ us-gaap_DefinedBenefitPlansDisclosuresDefinedBenefitPlansAxis
= us-gaap_PensionPlansDefinedBenefitMember

- Definition

Benefits paid and estimated expenses

+ References

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- Definition

Interest cost included in the net periodic benefit cost for the period.

+ References

No definition available.

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- Definition

The difference between fair value of plan assets at the end of the period and the fair value at the beginning of the period, adjusted for contributions and payments of benefits during the period, and after adjusting for taxes and other expenses, as applicable.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Actual Return on Plan Assets (Component of Net Periodic Postretirement Benefit Cost)

-URI http://asc.fasb.org/extlink&oid=6504192

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

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-SubTopic 20

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-Paragraph 1

-Subparagraph (b)(1)

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-Glossary Actual Return on Plan Assets (Component of Net Periodic Pension Cost)

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- Definition

Amount of gain (loss) related to change in benefit obligation resulting from changes in actuarial assumptions, for example, but not limited to, interest, mortality, employee turnover, salary, and temporary deviation from the substantive plan.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Gain or Loss

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- Definition

1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

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-Paragraph 1

-Subparagraph (a)

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-Glossary Projected Benefit Obligation

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Accumulated Postretirement Benefit Obligation

-URI http://asc.fasb.org/extlink&oid=6503904

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- Definition

The amount of payments made for which participants are entitled under a pension plan, including pension benefits, death benefits, and benefits due on termination of employment. Also includes payments made under a postretirement benefit plan, including prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services. This item represents a periodic decrease to the plan obligations and a decrease to plan assets.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

-Glossary Postretirement Benefits

-URI http://asc.fasb.org/extlink&oid=6521376

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- Definition

The increase in the fair value of plan assets from contributions made by the employer.

+ References

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- Definition

Assets, usually stocks, bonds, and other investments, that have been segregated and restricted (usually in a trust) to provide benefits, at their fair value as of the measurement date. Plan assets include amounts contributed by the employer (and by employees for a contributory plan) and amounts earned from investing the contributions, less benefits paid. If a plan has liabilities other than for benefits, those non-benefit obligations may be considered as reductions of plan assets.

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- Definition

The funded status is measured as the difference between the fair value of plan assets and the benefit obligation. Will normally be the same as the net Defined Benefit Plan, Amounts Recognized in Balance Sheet, Total.

+ References

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

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