Summary of Significant Accounting Policies Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015
Accumulated Other Comprehensive Income (Loss), Net of Tax	$ (28.7)	$ (34.7)	$ (29.3)	$ (35.1)
Amounts reclassified from accumulated other comprehensive income (loss)	0.6	0.4
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	(31.5)	(38.4)	(32.1)	(39.2)
Amounts reclassified from accumulated other comprehensive income (loss)	0.6	0.8
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member] | Other Postretirement Benefit Plan [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	2.7	2.5	2.7	2.5
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	0.1	0.1	0.1	0.1
Amounts reclassified from accumulated other comprehensive income (loss)	0.0	0.0
Accumulated Defined Benefit Plans Adjustment, Net Prior Service Cost (Credit) [Member] | Other Postretirement Benefit Plan [Member]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans, Net of Tax	0.0	1.1	$ 0.0	$ 1.5
Amounts reclassified from accumulated other comprehensive income (loss)	$ 0.0	$ (0.4)