Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax		$ (20.3)	$ (29.0)	$ (20.3)	$ (29.0)	$ (29.3)	$ (35.1)
Net current period other comprehensive income				9.0	6.1
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				6.7
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				1.8	1.1
Other Postretirement Benefits Plan [Member]
Other Comprehensive Income (Loss), Defined Benefit Plan, Transition Asset (Obligation), Reclassification Adjustment from AOCI, before Tax	[1]	0.7	0.0	0.7	0.0
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax		(30.1)	(31.9)	(30.1)	(31.9)	(32.1)	(39.2)
Net current period other comprehensive income				2.0	7.3
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				2.0	2.3
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax		3.2	2.5	3.2	2.5	2.7	2.5
Net current period other comprehensive income				0.5	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				0.0	0.0
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax		0.1	0.1	0.1	0.1	0.1	0.1
Net current period other comprehensive income				0.0	0.0
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				0.0	0.0
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefits Plan [Member]
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax		$ 6.5	$ 0.3	6.5	0.3	$ 0.0	$ 1.5
Net current period other comprehensive income				6.5	(1.2)
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				(0.2)	(1.2)
Defined Benefit Plans Income Loss [Member] | Pension Plan [Member]
Other Comprehensive Income (Loss), before Reclassifications, before Tax				0.0
Defined Benefit Plans Income Loss [Member] | Other Postretirement Benefits Plan [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				0.0
Defined Benefit Plan Prior Service Cost [Member] | Other Postretirement Benefits Plan [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				6.7
Pension Plan [Member] | Defined Benefit Plan Prior Service Cost [Member]
Other Comprehensive Income (Loss), before Reclassifications, Net of Tax				0.0
Settlement Cost [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				0.5	5.0
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				0.0	5.0
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefits Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				0.5	0.0
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax					0.0
Settlement Cost [Member] | Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefits Plan [Member]
Reclassification from Accumulated Other Comprehensive Income, Current Period, Net of Tax				$ 0.0	$ 0.0

[1] These accumulated other comprehensive income (loss) components are included in the computation of net periodic benefit cost (see Note 10 for additional information).