Employee Benefits Employee Benefits, Benefit obligations and funded status of our Canadian pension and the VEBAs (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Mar. 31, 2016	Mar. 31, 2015
Change in plan assets:
Fair value of plan assets at beginning of year	$ 69.1
Fair value of plan assets at end of year	69.6	$ 69.1
Accrued Annual Veba Contribution	20.0	19.6
Net liabilities of Salaried VEBA	28.6	19.0
Long-term portion of contingent contribution to Union VEBA	12.8	29.9
Canadian pension plan
Change in benefit obligation:
Obligation at beginning of year	6.1	7.0
Defined Benefit Plan, Foreign Currency Exchange Rate Gain (Loss)	0.2	(1.0)
Defined Benefit Plan, Service Cost at Balance Sheet Exchange Rate	0.3	0.2
Defined Benefit Plan, Interest Cost at Balance Sheet Exchange Rate	0.3	0.2
Defined Benefit Plan, Actuarial Gain (Loss)	(0.3)	0.1
Defined Benefit Plan, Benefits Paid	0.2	0.2
Obligation at end of year	7.0	6.1
Change in plan assets:
Fair value of plan assets at beginning of year	5.7	6.3
Defined Benefit Plan, Foreign Currency Exchange Rate Changes, Plan Assets	0.2	(1.0)
Defined Benefit Plan, Actual Return on Plan Assets	0.1	0.3
Defined Benefit Plan, Contributions by Employer	0.3	0.3
Defined Benefit Plan, Benefits Paid	0.2	0.2
Fair value of plan assets at end of year	6.1	5.7
Defined Benefit Plan, Funded Status of Plan	(0.9)	(0.4)
VEBAs
Change in benefit obligation:
Obligation at beginning of year	77.9	470.9
Defined Benefit Plan, Foreign Currency Exchange Rate Gain (Loss)	0.0	0.0
Defined Benefit Plan, Service Cost	0.0	0.0
Defined Benefit Plan, Interest Cost	2.9	2.7
Defined Benefit Plan, Plan Amendments	8.4	13.2
Defined Benefit Plan, Actuarial Gain (Loss)	(4.1)	11.2
Benefits paid by VEBA	(6.5)	(6.2)
Obligation at end of year	86.8	77.9
Change in plan assets:
Fair value of plan assets at beginning of year	58.9	793.8
Defined Benefit Plan, Actual Return on Plan Assets	2.9	0.1
Defined Benefit Plan, Contributions by Employer	2.9	49.5
Benefits paid by VEBA	(6.5)	(6.2)
Fair value of plan assets at end of year	58.2	58.9
Defined Benefit Plan, Funded Status of Plan	(28.6)	(19.0)
Salaried VEBA
Change in plan assets:
Actuarial (gain) loss due to projected lower benefit utilization	(0.4)	(5.5)
Actuarial (Gain) Loss Due To Change In Discount Rate	2.2	(2.0)
Actuarial (Gain) Loss Due To Changes In Census Data	2.3	(3.7)
Accrued Annual Veba Contribution	2.9	2.8
Defined Benefit Plan, Funded Status of Plan	(28.6)	(19.0)
Union VEBA
Change in benefit obligation:
Defined Benefit Plan, Settlements, Benefit Obligation	0.0	(391.5)
Change in plan assets:
Defined Benefit Plan, Settlements, Plan Assets	0.0	(778.3)
Accrued Annual Veba Contribution	17.1	$ 16.8	$ 16.7
Long-term portion of contingent contribution to Union VEBA	$ 12.8
Accrued Veba contingent contribution - total				$ 46.7