Guarantee of Securities of Subsidiaries Statements of Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Condensed Financial Statements, Captions [Line Items]
Net income (loss)	$ 69.4	$ 42.5	$ 75.7	$ 108.1	$ 63.0	$ 37.9	$ 42.2	$ 97.2	$ 295.7	$ 240.3	$ 297.0
Other comprehensive income (loss):
(Loss) gain on cash flow hedges									0.0	0.0	(1.5)
Reclassification adjustment transferred to Net income from cash flow hedges									0.9	1.2	2.5
Pension and other postretirement benefit costs, net of tax									3.2	(5.4)	(1.9)
Total Comprehensive Income (Loss)									299.8	236.1	296.1
Reportable Legal Entities | Parent Guarantor
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									295.7	240.3	297.0
Other comprehensive income (loss):
(Loss) gain on cash flow hedges											(1.5)
Reclassification adjustment transferred to Net income from cash flow hedges									0.9	1.2	2.5
Pension and other postretirement benefit costs, net of tax									3.2	(5.4)	(1.9)
Total Comprehensive Income (Loss)									299.8	236.1	296.1
Reportable Legal Entities | Subsidiary Issuer
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									227.2	172.8	250.0
Other comprehensive income (loss):
(Loss) gain on cash flow hedges											(1.5)
Reclassification adjustment transferred to Net income from cash flow hedges									0.9	1.2	2.5
Pension and other postretirement benefit costs, net of tax									3.2	(5.4)	(1.9)
Total Comprehensive Income (Loss)									231.3	168.6	249.1
Reportable Legal Entities | Non-Guarantor Subsidiaries
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									415.0	349.1	419.3
Other comprehensive income (loss):
(Loss) gain on cash flow hedges											0.0
Reclassification adjustment transferred to Net income from cash flow hedges									0.7	0.7	0.7
Pension and other postretirement benefit costs, net of tax									3.2	(5.4)	(1.9)
Total Comprehensive Income (Loss)									418.9	344.4	418.1
Eliminations
Condensed Financial Statements, Captions [Line Items]
Net income (loss)									(642.2)	(521.9)	(669.3)
Other comprehensive income (loss):
(Loss) gain on cash flow hedges											1.5
Reclassification adjustment transferred to Net income from cash flow hedges									(1.6)	(1.9)	(3.2)
Pension and other postretirement benefit costs, net of tax									(6.4)	10.8	3.8
Total Comprehensive Income (Loss)									$ (650.2)	$ (513.0)	$ (667.2)