Fair Value of Pension Plan Assets (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	$ 503.0	$ 461.1
Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	14.7	14.4
Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	29.5	28.0
Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	23.4	22.9
Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	7.9	7.8
Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	27.7	20.1
Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	103.2	93.2
Investments measured at net asset value
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	399.8	367.9
Level 1 | Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	14.7	14.4
Level 1 | Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 1 | Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	23.4	22.6
Level 1 | Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 1 | Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	27.7	20.1
Level 1 | Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	65.8	57.1
Level 2 | Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 2 | Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 2 | Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.3
Level 2 | Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	7.9	7.8
Level 2 | Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 2 | Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	7.9	8.1
Level 3 | Investment funds
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 3 | Limited partnerships
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	29.5	28.0
Balance at beginning of period	28.0	31.0
Actual return on plan assets	1.5	(3.0)
Balance at end of period	29.5	28.0
Level 3 | Fixed income securities
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 3 | Insurance contracts
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 3 | Other plan assets
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	0.0	0.0
Level 3 | Total plan assets subject to leveling
Fair value assets and liabilities measured on recurring basis
Defined Benefit Plan, Fair Value of Plan Assets	$ 29.5	$ 28.0