Pension and Postretirement Plans - Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 488.2	$ 399.6
U.S. equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	165.0	151.0
International equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	127.2	101.3
Fixed income bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	183.0	136.1
Fixed income short-term money market funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	7.2	3.7
Group insurance contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5.8	7.5
Quoted Prices in active markets Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	327.3	392.1
Quoted Prices in active markets Level 1 | U.S. equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	107.2	151.0
Quoted Prices in active markets Level 1 | International equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	100.7	101.3
Quoted Prices in active markets Level 1 | Fixed income bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	119.3	136.1
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	0.1	3.7
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	155.1	0.0
Significant observable inputs Level 2 | U.S. equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	57.8	0.0
Significant observable inputs Level 2 | International equity index funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	26.5	0.0
Significant observable inputs Level 2 | Fixed income bond funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	63.7	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	7.1	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