Pension and Other Postretirement Benefit Plans - Recurring Fair Value Measures (Details) - USD ($) $ in Millions	Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Pension [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	$ 557.5		$ 521.3		$ 544.2
Pension [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	159.1		144.1
Pension [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	332.2		305.0
Pension [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	66.2		72.2
Pension [Member] | U.S. Large-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	94.6	[1]	76.0	[2]
Pension [Member] | U.S. Large-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	94.6	[1]	33.9	[2]
Pension [Member] | U.S. Large-cap [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	42.1	[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	44.8	[1]	31.9	[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]	14.2	[2]
Pension [Member] | U.S. Mid-cap Growth [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	44.8	[1]	17.7	[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	45.0	[1]	32.4	[2]
Pension [Member] | U.S. Small-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0	[1]	14.5	[2]
Pension [Member] | U.S. Small-cap [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	45.0	[1]	17.9	[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] | Mutual Funds [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			8.4
Pension [Member] | Mutual Funds [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			8.4
Pension [Member] | Mutual Funds [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			0.0
Pension [Member] | Mutual Funds [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			0.0
Pension [Member] | International [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	89.0		86.7
Pension [Member] | International [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	46.7		44.7
Pension [Member] | International [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	42.3		42.0
Pension [Member] | International [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Pension [Member] | Debt Securities - Mutual Funds [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			0.1
Pension [Member] | Debt Securities - Mutual Funds [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			0.1
Pension [Member] | Debt Securities - Mutual Funds [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			0.0
Pension [Member] | Debt Securities - Mutual Funds [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets			0.0
Pension [Member] | Debt Securities - Fixed Income [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	200.1		188.0
Pension [Member] | Debt Securities - Fixed Income [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		2.7
Pension [Member] | Debt Securities - Fixed Income [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	200.1		185.3
Pension [Member] | Debt Securities - 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	17.8		25.6
Pension [Member] | Cash and Cash Equivalents [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	17.8		25.6
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	40.6		43.3
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	40.6		43.3
Pension [Member] | Real Estate [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	25.6		28.9
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	25.6		28.9
Postretirement Health and Life [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	154.3		153.4		$ 163.2
Postretirement Health and Life [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	140.2		133.0
Postretirement Health and Life [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	4.6		8.4
Postretirement Health and Life [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	9.5		12.0
Postretirement Health and Life [Member] | U.S. Large-cap [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	27.9	[3]	28.2	[4]
Postretirement Health and Life [Member] | U.S. Large-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	27.9	[3]	28.2	[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	20.7	[3]	19.1	[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	20.7	[3]	19.1	[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	14.0	[3]	12.1	[4]
Postretirement Health and Life [Member] | U.S. Small-cap [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	14.0	[3]	12.1	[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	27.9		26.8
Postretirement Health and Life [Member] | International [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	27.9		26.8
Postretirement Health and Life [Member] | International [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | International [Member] | Level 3 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	0.0		0.0
Postretirement Health and Life [Member] | Debt Securities - Mutual Funds [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	48.6		45.2
Postretirement Health and Life [Member] | Debt Securities - Mutual Funds [Member] | Level 1 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	48.6		45.2
Postretirement Health and Life [Member] | Debt Securities - 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] | Debt Securities - 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] | Debt Securities - Fixed Income [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	4.6		8.4
Postretirement Health and Life [Member] | Debt Securities - 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] | Debt Securities - Fixed Income [Member] | Level 2 [Member]
Recurring Fair Value Measures [Line Items]
Fair Value of Assets	4.6		8.4
Postretirement Health and Life [Member] | Debt Securities - 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		1.6
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		1.6
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	9.5		12.0
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	$ 9.5		$ 12.0

[1] The underlying investments classified under U.S. Equity Securities consist of money market funds (Level 1), mutual funds (Level 1) and actively-managed funds (Level 2), which are combined with futures, and settle daily, to achieve the returns of the U.S. Equity Securities Mid-cap Growth and Small-cap funds. Our exposure with respect to these investments includes both the futures and the underlying investments.

[2] The underlying investments classified under U.S. Equity Securities consist of money market funds (Level 1) and actively-managed funds (Level 2), which are combined with futures, and settle daily, to achieve the returns of the U.S. Equity Securities Large-cap, Mid-cap Growth, and Small-cap funds. Our exposure with respect to these investments includes both the futures and the underlying investments.

[3] The underlying investments classified under U.S. Equity Securities consist of mutual funds (Level 1).

[4] The underlying investments classified under U.S. Equity Securities consist of mutual funds (Level 1).