Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	$ 508.6	$ 453.7
Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	18.0	13.0
Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	25.6	25.6
Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	19.6	23.1
Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	5.5	5.8
Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	27.8	20.0
Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	96.5	87.5
Investments measured at net asset value
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	412.1	366.2
Level 1 | Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	18.0	13.0
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	19.6	23.1
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	27.8	20.0
Level 1 | Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Plan Assets, Amount	65.4	56.1
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	5.5	5.8
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	5.5	5.8
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	25.6
Balance at beginning of period	25.6	29.5
Actual return on plan assets	0.0	(3.9)
Balance at end of period	25.6	25.6
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	$ 25.6