Pension and Savings Plans - Fair Value of Assets in the Master Trust by Asset (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Level 3
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	$ 1.2	$ 4.1	$ 6.2	$ 11.8
Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	847.5	855.2	$ 764.8
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	805.4	812.1
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	623.3	598.3
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	142.6	154.9
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | International government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	18.7	18.2
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	4.7	3.4
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	14.9	33.2
Pension Plan | Fair Value, Inputs, Level 1, 2 and 3 | Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	1.2	4.1
Pension Plan | Level 1
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	136.3	169.1
Pension Plan | Level 1 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 1 | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	121.4	135.9
Pension Plan | Level 1 | International government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 1 | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 1 | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	14.9	33.2
Pension Plan | Level 1 | Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 2
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	667.9	638.9
Pension Plan | Level 2 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	623.3	598.3
Pension Plan | Level 2 | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	21.2	19.0
Pension Plan | Level 2 | International government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	18.7	18.2
Pension Plan | Level 2 | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	4.7	3.4
Pension Plan | Level 2 | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 2 | Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 3
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	1.2	4.1
Pension Plan | Level 3 | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 3 | U.S. government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 3 | International government securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 3 | Asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 3 | Cash funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	0.0	0.0
Pension Plan | Level 3 | Real estate interests
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	1.2	4.1
Pension Plan | Fair Value Measured at Net Asset Value Per Share | Private mutual funds
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, assets	$ 42.1	$ 43.1