Retirement Benefits - Narrative (Details) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) plan	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Number of Defined Benefit Pension Plans | plan	2
Defined Benefit Plan Benefit Calculation Condition Period	5 years
Defined Benefit Plan Vesting Condition Period	2 years
Accumulated benefit obligation	$ 57.6	$ 46.9
Estimated amortization from AOCI of net actuarial losses	0.4
Defined Benefit Plan, Expected Future Employer Contributions, Next Fiscal Year	4.1
Multiemployer Plan, Contributions by Employer	4.0	3.8	$ 3.3
Defined Contribution Plan, employer contributions	$ 5.9	$ 5.5	$ 4.8
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Plan Asset Allocations	32.00%
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Plan Asset Allocations	60.00%