Retirement Plans & Other Postretirement Benefits - Fair Values of Pension and Other Retirement Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	$ 316.1	$ 304.5
Domestic equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	140.2	135.1
International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	46.8	42.0
Bond funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	43.6	44.6
Real estate, commodity & other funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	10.7	10.4
Investments measured at net asset value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	74.8	72.4
Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	236.8	227.7
Level 1 | Domestic equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	140.2	135.1
Level 1 | International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	46.8	42.0
Level 1 | Bond funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	43.6	44.6
Level 1 | Real estate, commodity & other funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	6.2	6.0
Level 1 | Investments measured at net asset value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 2 | Domestic equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 2 | International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 2 | Bond funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 2 | Real estate, commodity & other funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 2 | Investments measured at net asset value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	4.5	4.4
Level 3 | Domestic equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 3 | International equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 3 | Bond funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	0.0	0.0
Level 3 | Real estate, commodity & other funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	4.5	4.4
Level 3 | Investments measured at net asset value
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets, end of period	$ 0.0	$ 0.0