Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	$ 453.7	$ 503.0
Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	13.0	14.7
Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	25.6	29.5
Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	23.1	23.4
Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	5.8	7.9
Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	20.0	27.7
Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	87.5	103.2
Investments measured at net asset value
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	366.2	399.8
Level 1 | Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	13.0	14.7
Level 1 | Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 1 | Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	23.1	23.4
Level 1 | Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 1 | Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	20.0	27.7
Level 1 | Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	56.1	65.8
Level 2 | Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 2 | Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 2 | Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 2 | Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	5.8	7.9
Level 2 | Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 2 | Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	5.8	7.9
Level 3 | Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 3 | Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	25.6	29.5
Balance at beginning of period	29.5	28.0
Actual return on plan assets	(3.9)	1.5
Balance at end of period	25.6	29.5
Level 3 | Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 3 | Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 3 | Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	0.0	0.0
Level 3 | Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	$ 25.6	$ 29.5