Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures (Details) - USD ($) $ in Millions	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Pension [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	$ 598.0		$ 628.2		$ 557.5
Pension [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	6.3		12.4
Pension [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	543.1		557.1
Pension [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	48.6		58.7
Pension [Member] | U.S. Large-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	59.1	[1]	108.6	[2]
Pension [Member] | U.S. Large-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | U.S. Large-cap [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	59.1	[1]	108.6	[2]
Pension [Member] | U.S. Large-cap [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | U.S. Mid-cap Growth [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	28.1	[1]	51.9	[2]
Pension [Member] | U.S. Mid-cap Growth [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | U.S. Mid-cap Growth [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	28.1	[1]	51.9	[2]
Pension [Member] | U.S. Mid-cap Growth [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | U.S. Small-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	27.2	[1]	51.5	[2]
Pension [Member] | U.S. Small-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | U.S. Small-cap [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	27.2	[1]	51.5	[2]
Pension [Member] | U.S. Small-cap [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | International [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	75.8		122.3
Pension [Member] | International [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | International [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	75.8		122.3
Pension [Member] | International [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Fixed Income [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	352.9		222.8
Pension [Member] | Fixed Income [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Fixed Income [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	352.9		222.8
Pension [Member] | Fixed Income [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Cash and Cash Equivalents [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	6.3		12.4
Pension [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	6.3		12.4
Pension [Member] | Cash and Cash Equivalents [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Cash and Cash Equivalents [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Private Equity Funds [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	27.8		33.2
Pension [Member] | Private Equity Funds [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Private Equity Funds [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Private Equity Funds [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	27.8		33.2
Pension [Member] | Real Estate [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	20.8		25.5
Pension [Member] | Real Estate [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Real Estate [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Real Estate [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	20.8		25.5
Postretirement Health and Life [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	154.3		171.0		$ 154.3
Postretirement Health and Life [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	143.8		158.3
Postretirement Health and Life [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	4.0		4.5
Postretirement Health and Life [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	6.5		8.2
Postretirement Health and Life [Member] | U.S. Large-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	29.1	[3]	32.1	[4]
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	29.1	[3]	32.1	[4]
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	21.2	[3]	24.3	[4]
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	21.2	[3]	24.3	[4]
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | U.S. Mid-cap Growth [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | U.S. Small-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	12.9	[3]	15.5	[4]
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	12.9	[3]	15.5	[4]
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | International [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	30.4	[3]	35.8	[4]
Postretirement Health and Life [Member] | International [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	30.4	[3]	35.8	[4]
Postretirement Health and Life [Member] | International [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | International [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[3]	0.0	[4]
Postretirement Health and Life [Member] | Mutual Funds [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	49.6		49.8
Postretirement Health and Life [Member] | Mutual Funds [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	49.6		49.8
Postretirement Health and Life [Member] | Mutual Funds [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Mutual Funds [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Fixed Income [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	4.0		4.5
Postretirement Health and Life [Member] | Fixed Income [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Fixed Income [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	4.0		4.5
Postretirement Health and Life [Member] | Fixed Income [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.6		0.8
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.6		0.8
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Private Equity Funds [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	6.5		8.2
Postretirement Health and Life [Member] | Private Equity Funds [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Private Equity Funds [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Private Equity Funds [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	$ 6.5		$ 8.2

[1] The underlying investments consist of actively-managed funds managed to achieve the returns of certain U.S. equity securities large‑cap, mid-cap and small-cap indexes. NOTE 15. PENSION AND OTHER POSTRETIREMENT BENEFIT PLANS (Continued)Fair Value (Continued)Recurring Fair Value Measures Activity in Level 3Private Equity Funds Real EstateMillions Balance as of December 31, 2017$33.2$25.5Actual Return on Plan Assets2.80.7Purchases, Sales, and Settlements – Net(8.2)(5.4)Balance as of December 31, 2018$27.8$20.8

[2] The underlying investments consist of actively-managed funds managed to achieve the returns of certain U.S. equity securities large‑cap, mid-cap and small-cap indexes. Recurring Fair Value Measures Activity in Level 3 Private Equity Funds Real EstateMillions Balance as of December 31, 2016 $40.6$25.6Actual Return on Plan Assets 7.11.7Purchases, Sales, and Settlements – Net (14.5)(1.8)Balance as of December 31, 2017 $33.2$25.5

[3] The underlying investments consist of mutual funds (Level 1).

[4] The underlying investments consist of mutual funds (Level 1).