Employee Benefits - Changes in the Fair Value of Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	$ 1,179.3	$ 1,160.0
Ending balance at December 31	1,102.8	1,179.3
Pension Benefits | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	4.5
Ending balance at December 31	11.0	4.5
OPEB
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	216.1	224.9
Ending balance at December 31	205.1	216.1
OPEB | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.3
Ending balance at December 31	0.7	0.3
Private Placement | Pension Benefits
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	4.5
Ending balance at December 31	11.0	4.5
Private Placement | Pension Benefits | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	4.5	0.0
Purchases	6.5	4.5
Ending balance at December 31	11.0	4.5
Private Placement | OPEB
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.3
Ending balance at December 31	0.7	0.3
Private Placement | OPEB | Level 3
Reconciliation of changes in the fair value of pension plan assets
Beginning balance at January 1	0.3	0.0
Purchases	0.4	0.3
Ending balance at December 31	$ 0.7	$ 0.3