Guarantees (Details) $ in Millions	Sep. 30, 2015 USD ($)
Guarantees
Total guarantees	$ 297.8
Guarantees expiring in less than 1 year	175.2
Guarantees expiring within 1 to 3 years	0.2
Guarantees with expiration over 3 years	122.4
Guarantees supporting commodity transactions of subsidiaries
Guarantees
Total guarantees	161.4
Guarantees expiring in less than 1 year	89.4
Guarantees expiring within 1 to 3 years	0.0
Guarantees with expiration over 3 years	72.0
Guarantees supporting commodity transactions of subsidiaries | WBS
Guarantees
Total guarantees	5.0
Guarantees supporting commodity transactions of subsidiaries | PDL
Guarantees
Total guarantees	6.0
Guarantees supporting commodity transactions of subsidiaries | ITF
Guarantees
Total guarantees	0.9
Guarantees supporting commodity transactions of subsidiaries | MERC
Guarantees
Total guarantees	113.8
Guarantees supporting commodity transactions of subsidiaries | MGU
Guarantees
Total guarantees	35.7
Standby letters of credit
Guarantees
Total guarantees	28.6
Guarantees expiring in less than 1 year	28.5
Guarantees expiring within 1 to 3 years	0.1
Guarantees with expiration over 3 years	0.0
Surety bonds
Guarantees
Total guarantees	36.6
Guarantees expiring in less than 1 year	36.6
Guarantees expiring within 1 to 3 years	0.0
Guarantees with expiration over 3 years	0.0
Other guarantees
Guarantees
Total guarantees	71.2
Guarantees expiring in less than 1 year	20.7
Guarantees expiring within 1 to 3 years	0.1
Guarantees with expiration over 3 years	50.4
Other guarantees | PDL
Guarantees
Total guarantees	19.1
Other guarantees | ITF
Guarantees
Total guarantees	11.2
Other guarantees | WPS
Guarantees
Total guarantees	20.0
Other guarantees | Integrys
Guarantees
Total guarantees	10.0
Reciprocal guarantee | PDL
Guarantees
Total guarantees	6.6
Other indemnifications
Guarantees
Total guarantees	10.9
Liability related to workers compensation coverage	$ 10.2