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Long-Term Debt and Capital Lease Obligations - Long-term Debt (Details) $ in Millions		1 Months Ended				12 Months Ended
	Oct. 15, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	May 31, 2018 USD ($)	Dec. 31, 2018 USD ($) number_of_interest_rate_swaps	Jul. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	May 15, 2017
Debt Disclosure [Abstract]
Number of interest rate swaps executed \| number_of_interest_rate_swaps						2
Future maturities of long-term debt outstanding
2019		$ 360.1				$ 360.1
2020		686.9				686.9
2021		1,338.8				1,338.8
2022		40.8				40.8
2023		42.8				42.8
Thereafter		7,918.2				7,918.2
Total		10,387.6				10,387.6
WEC Energy Group
Debt Disclosure [Abstract]
Current portion of long-term debt		0.0				0.0		$ 300.0
Future maturities of long-term debt outstanding
2020		400.0				400.0
2021		600.0				600.0
Thereafter		1,200.0				1,200.0
Total		2,200.0				2,200.0
WEC Energy Group \| WEC 2007 Junior Notes due 2067
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 500.0				$ 500.0		$ 500.0
Debt instrument interest rate stated percentage rate		4.73%				4.73%		3.53%	6.25%
WEC Energy Group \| WEC 3.375% Senior Notes due June 15, 2021
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 600.0				$ 600.0		$ 0.0
Proceeds from issuance of debt				$ 600.0
Debt instrument interest rate stated percentage rate		3.375%		3.375%		3.375%
WEC Energy Group \| WEC Senior Notes due 2018
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 0.0				$ 0.0		300.0
Debt instrument interest rate stated percentage rate		1.65%		1.65%		1.65%
Extinguishment of debt				$ 300.0
WEC Energy Group \| WEC Energy Group senior notes (unsecured), 6.20% due 2033
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 200.0				$ 200.0		200.0
Debt instrument interest rate stated percentage rate		6.20%				6.20%
WE
Debt Disclosure [Abstract]
Long-term pollution control bond							$ 80.0
WE \| WE 4.30% Debentures due 2048
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 300.0				$ 300.0		0.0
Proceeds from issuance of debt	$ 300.0
Debt instrument interest rate stated percentage rate	4.30%	4.30%				4.30%
WE \| WE Debentures due 2018
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 0.0				$ 0.0		250.0
Debt instrument interest rate stated percentage rate		1.70%		1.70%		1.70%
Extinguishment of debt				$ 250.0
WPS \| WPS Senior Notes due November 21, 2021
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 400.0				$ 400.0		0.0
Proceeds from issuance of debt			$ 400.0
Debt instrument interest rate stated percentage rate		3.35%	3.35%			3.35%
WPS \| WPS Senior Notes (unsecured), 1.65% due 2018
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 0.0				$ 0.0		250.0
Debt instrument interest rate stated percentage rate		1.65%				1.65%
Extinguishment of debt		$ 250.0
PGL \| PGL Series FFF Bonds due 2028
Debt Disclosure [Abstract]
Proceeds from issuance of debt			$ 150.0
Debt instrument interest rate stated percentage rate		3.87%	3.87%			3.87%
PGL \| PGL First and Refunding Mortgage Bonds (secured), 8.00% due 2018
Debt Disclosure [Abstract]
Debt instrument interest rate stated percentage rate		8.00%	8.00%			8.00%
Extinguishment of debt			$ 5.0
NSG \| NSG Series R Bonds due 2028 [Member]
Debt Disclosure [Abstract]
Proceeds from issuance of debt			$ 50.0
Debt instrument interest rate stated percentage rate		3.87%	3.87%			3.87%
We Power \| We Power Subsidiaries Notes (secured, nonrecourse), 4.91% due 2019-2030
Debt Disclosure [Abstract]
Debt instrument interest rate stated percentage rate		4.91%				4.91%
Current portion of long-term debt		$ 6.2				$ 6.2
Long-term debt, gross		$ 95.1				$ 95.1
We Power \| We Power Subsidiaries Notes (secured, nonrecourse), 6.00% due 2019-2033
Debt Disclosure [Abstract]
Debt instrument interest rate stated percentage rate		6.00%				6.00%
Current portion of long-term debt		$ 5.2				$ 5.2
Long-term debt, gross		$ 116.6				$ 116.6
We Power \| We Power Subsidiaries Notes (secured, nonrecourse), 5.209% due 2019-2030
Debt Disclosure [Abstract]
Debt instrument interest rate stated percentage rate		5.209%				5.209%
Current portion of long-term debt		$ 12.0				$ 12.0
Long-term debt, gross		$ 182.7				$ 182.7
We Power \| We Power Subsidiaries Notes (secured, nonrecourse), 4.673% due 2019-2031
Debt Disclosure [Abstract]
Debt instrument interest rate stated percentage rate		4.673%				4.673%
Current portion of long-term debt		$ 9.3				$ 9.3
Long-term debt, gross		153.5				153.5
Integrys Holding Inc \| Integrys 2006 Junior Notes due 2066
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 0.0				$ 0.0		114.9
Debt instrument interest rate stated percentage rate		3.60%				3.60%
Extinguishment of debt					$ 114.9
Integrys Holding Inc \| Integrys junior notes (unsecured), 6.00% due 2073
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 400.0				$ 400.0		400.0
Debt instrument interest rate stated percentage rate		6.00%				6.00%
Debt instrument, basis spread on variable rate						3.22%
Long-term debt, gross		$ 400.0				$ 400.0
Bluewater Gas Storage, LLC \| Bluewater Gas Storage senior notes (unsecured), 3.76% due 2047
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 122.7				$ 122.7		125.0
Debt instrument interest rate stated percentage rate		3.76%				3.76%
Current portion of long-term debt		$ 2.4				$ 2.4
Long-term debt, gross		122.7				122.7
ATC Holding LLC \| ATC Holding Total Senior Notes issued December 2018
Debt Disclosure [Abstract]
Proceeds from issuance of debt		240.0
ATC Holding LLC \| ATC Holding 4.18% Senior Notes due December 20, 2025
Debt Disclosure [Abstract]
Long-term debt outstanding		85.0				$ 85.0		0.0
Proceeds from issuance of debt		$ 85.0
Debt instrument interest rate stated percentage rate		4.18%				4.18%
ATC Holding LLC \| ATC Holding 4.37% Senior Notes due December 20, 2028
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 56.5				$ 56.5		0.0
Proceeds from issuance of debt		$ 56.5
Debt instrument interest rate stated percentage rate		4.37%				4.37%
ATC Holding LLC \| ATC Holding 4.47% Senior Notes due December 20, 2030
Debt Disclosure [Abstract]
Long-term debt outstanding		$ 98.5				$ 98.5		$ 0.0
Proceeds from issuance of debt		$ 98.5
Debt instrument interest rate stated percentage rate		4.47%				4.47%
Interest rate swaps \| WEC Energy Group
Debt Disclosure [Abstract]
Number of interest rate swaps executed \| number_of_interest_rate_swaps						2
Interest rate swap notional value		$ 250.0				$ 250.0	$ 250.0
Interest rate swap fixed interest rate		4.9765%				4.9765%