Guarantee of Securities of Subsidiaries Guarantee of Securities of Subsidiaries Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 69.8	$ 47.3	$ 212.8	$ 214.0
Other comprehensive income (loss):
Loss on cash flow hedge	0.0	0.0	(1.5)	0.0
Reclassification adjustment transferred to Net income from cash flow hedges	0.6	0.6	1.9	1.8
Pension and other postretirement benefit costs	0.1	0.6	0.8	2.3
Total Comprehensive Income (Loss)	70.5	48.5	214.0	218.1
Reportable Legal Entities | Parent Guarantor
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	69.8	47.3	212.8	214.0
Other comprehensive income (loss):
Loss on cash flow hedge			(1.5)
Reclassification adjustment transferred to Net income from cash flow hedges	0.6	0.6	1.9	1.8
Pension and other postretirement benefit costs	0.1	0.6	0.8	2.3
Total Comprehensive Income (Loss)	70.5	48.5	214.0	218.1
Reportable Legal Entities | Subsidiary Issuer
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	57.5	37.7	178.4	187.0
Other comprehensive income (loss):
Loss on cash flow hedge			(1.5)
Reclassification adjustment transferred to Net income from cash flow hedges	0.6	0.6	1.9	1.8
Pension and other postretirement benefit costs	0.1	0.6	0.8	2.3
Total Comprehensive Income (Loss)	58.2	38.9	179.6	191.1
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	100.7	84.0	305.9	313.3
Other comprehensive income (loss):
Loss on cash flow hedge			0.0
Reclassification adjustment transferred to Net income from cash flow hedges	0.2	0.2	0.5	0.6
Pension and other postretirement benefit costs	0.1	0.6	0.8	2.3
Total Comprehensive Income (Loss)	101.0	84.8	307.2	316.2
Eliminations
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	(158.2)	(121.7)	(484.3)	(500.3)
Other comprehensive income (loss):
Loss on cash flow hedge			1.5
Reclassification adjustment transferred to Net income from cash flow hedges	(0.8)	(0.8)	(2.4)	(2.4)
Pension and other postretirement benefit costs	(0.2)	(1.2)	(1.6)	(4.6)
Total Comprehensive Income (Loss)	$ (159.2)	$ (123.7)	$ (486.8)	$ (507.3)