Pension and Postretirement Plans - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 399.6	$ 381.0
U.S. equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151.0	142.7
International equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101.3	95.6
Fixed income bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136.1	123.9
Fixed income short-term money market funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.7	10.1
Group insurance contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.5	8.7
Quoted Prices in active markets Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	392.1	372.3
Quoted Prices in active markets Level 1 | U.S. equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151.0	142.7
Quoted Prices in active markets Level 1 | International equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101.3	95.6
Quoted Prices in active markets Level 1 | Fixed income bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136.1	123.9
Quoted Prices in active markets Level 1 | Fixed income short-term money market funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3.7	10.1
Quoted Prices in active markets Level 1 | Group insurance contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant observable inputs Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant observable inputs Level 2 | U.S. equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant observable inputs Level 2 | International equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant observable inputs Level 2 | Fixed income bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant observable inputs Level 2 | Fixed income short-term money market funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant observable inputs Level 2 | Group insurance contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs Level 3 | U.S. equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs Level 3 | International equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs Level 3 | Fixed income bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs Level 3 | Fixed income short-term money market funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0.0	0.0
Significant unobservable inputs Level 3 | Group insurance contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0.0	$ 0.0