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Commitments and Contingencies - Unconditional Purchase Obligations (Details) $ in Millions	Dec. 31, 2017 USD ($)
Minimum future commitments for purchase obligations
Total Amounts Committed	$ 11,640.9
2018	1,084.2
2019	885.5
2020	805.9
2021	695.4
2022	674.3
Later Years	7,495.6
Nuclear | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	9,184.5
2018	420.1
2019	445.4
2020	475.1
2021	501.1
2022	531.2
Later Years	6,811.6
Purchased power | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	645.3
2018	109.3
2019	73.5
2020	72.8
2021	68.9
2022	62.1
Later Years	258.7
Coal supply and transportation | Electric
Minimum future commitments for purchase obligations
Total Amounts Committed	341.2
2018	223.3
2019	72.0
2020	38.8
2021	2.1
2022	2.1
Later Years	2.9
Natural gas utility supply and transportation | Natural gas
Minimum future commitments for purchase obligations
Total Amounts Committed	1,469.9
2018	331.5
2019	294.6
2020	219.2
2021	123.3
2022	78.9
Later Years	$ 422.4

X

- Definition

Minimum amount of purchase arrangement in which the entity has agreed to expend funds to procure goods or services from a supplier.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Minimum amount of purchase arrangement maturing after the fifth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueAfterFifthYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Minimum amount of purchase arrangement maturing in the fifth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInFifthYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Minimum amount of purchase arrangement maturing in the fourth fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInFourthYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Minimum amount of purchase arrangement maturing in the next fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Minimum amount of purchase arrangement maturing in the second fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInSecondYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Minimum amount of purchase arrangement maturing in the third fiscal year following the latest fiscal year. Includes, but not limited to, recorded and unrecorded purchase obligations, long-term purchase commitments, and short-term purchase commitments.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInThirdYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- References

No definition available.

+ Details

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

X

- Details

Name:	us-gaap_SupplyCommitmentAxis=wec_NuclearMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ProductOrServiceAxis=wec_ElectricOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_SupplyCommitmentAxis=wec_PurchasedPowerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_SupplyCommitmentAxis=wec_CoalSupplyAndTransportationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_SupplyCommitmentAxis=wec_NaturalGasSupplyAndTransportationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ProductOrServiceAxis=wec_NaturalGasOperationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: