Guarantees (Details) $ in Millions	Dec. 31, 2015 USD ($)
Guarantor Obligations [Line Items]
Total guarantees	$ 312.0
Guarantees expiring in less than one year	172.7
Guarantees expiring within one to three years	9.8
Guarantees with expiration over three years	129.5
Guarantees supporting commodity transactions of subsidiaries
Guarantor Obligations [Line Items]
Total guarantees	174.5
Guarantees expiring in less than one year	95.0
Guarantees expiring within one to three years	0.0
Guarantees with expiration over three years	79.5
Guarantees supporting commodity transactions of subsidiaries | WBS
Guarantor Obligations [Line Items]
Total guarantees	5.0
Guarantees supporting commodity transactions of subsidiaries | PDL
Guarantor Obligations [Line Items]
Total guarantees	11.0
Guarantees supporting commodity transactions of subsidiaries | ITF
Guarantor Obligations [Line Items]
Total guarantees	0.6
Guarantees supporting commodity transactions of subsidiaries | MERC
Guarantor Obligations [Line Items]
Total guarantees	117.6
Guarantees supporting commodity transactions of subsidiaries | MGU
Guarantor Obligations [Line Items]
Total guarantees	40.3
Standby letters of credit
Guarantor Obligations [Line Items]
Total guarantees	28.4
Guarantees expiring in less than one year	18.5
Guarantees expiring within one to three years	9.7
Guarantees with expiration over three years	0.2
Surety bonds
Guarantor Obligations [Line Items]
Total guarantees	38.6
Guarantees expiring in less than one year	38.6
Guarantees expiring within one to three years	0.0
Guarantees with expiration over three years	0.0
Other guarantees
Guarantor Obligations [Line Items]
Total guarantees	70.5
Guarantees expiring in less than one year	20.6
Guarantees expiring within one to three years	0.1
Guarantees with expiration over three years	49.8
Other guarantees | PDL
Guarantor Obligations [Line Items]
Total guarantees	19.1
Other guarantees | ITF
Guarantor Obligations [Line Items]
Total guarantees	11.2
Other guarantees | WPS
Guarantor Obligations [Line Items]
Total guarantees	20.0
Other guarantees | Integrys Holding Inc
Guarantor Obligations [Line Items]
Total guarantees	10.0
Reciprocal guarantee | PDL
Guarantor Obligations [Line Items]
Total guarantees	6.6
Other indemnification
Guarantor Obligations [Line Items]
Total guarantees	10.2
Liability related to workers compensation coverage	$ 9.6