Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures (Details) - USD ($) $ in Millions	Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Pension [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	$ 759.4		$ 699.6		$ 598.0
Pension [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	3.2		7.1
Pension [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	731.2		666.5
Pension [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	25.0		26.0
Pension [Member] | U.S. Large-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	91.7	[1]	78.5	[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	91.7	[1]	78.5	[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	40.0	[1]	35.9	[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	40.0	[1]	35.9	[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	40.7	[1]	34.6	[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	40.7	[1]	34.6	[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	97.1		92.1
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	97.1		92.1
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	461.7	[1]	425.4	[2]
Pension [Member] | Fixed Income [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | Fixed Income [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	461.7	[1]	425.4	[2]
Pension [Member] | Fixed Income [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	0.0	[2]
Pension [Member] | Cash and Cash Equivalents [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	3.2		7.1
Pension [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	3.2		7.1
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 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	7.0		8.0
Pension [Member] | Private Equity [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Private Equity [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Private Equity [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	7.0		8.0		27.8
Pension [Member] | Real Estate [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	18.0		18.0
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	18.0		18.0		20.8
Postretirement Health and Life [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	186.0		173.7		154.3
Postretirement Health and Life [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	182.1		167.9
Postretirement Health and Life [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	2.2		4.1
Postretirement Health and Life [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	1.7		1.7
Postretirement Health and Life [Member] | U.S. Large-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	34.2	[3]	33.6	[4]
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	34.2	[3]	33.6	[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	31.4	[3]	27.7	[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	31.4	[3]	27.7	[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	16.6	[3]	14.3	[4]
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	16.6	[3]	14.3	[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	41.5	[3]	37.8	[4]
Postretirement Health and Life [Member] | International [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	41.5	[3]	37.8	[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	57.3		53.4
Postretirement Health and Life [Member] | Mutual Funds [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	57.3		53.4
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	2.2		4.1
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	2.2		4.1
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	1.1		1.1
Postretirement Health and Life [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	1.1		1.1
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 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	1.7		1.7
Postretirement Health and Life [Member] | Private Equity [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 [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 [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	$ 1.7		$ 1.7		$ 6.5

[1] The underlying investments consist of actively-managed funds managed to achieve the returns of certain U.S. equity and fixed income securities indexes.

[2] The underlying investments consist of actively-managed funds managed to achieve the returns of certain U.S. equity and fixed income securities indexes.

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

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