Pension and Other Postretirement Benefits - Expected Contribution Cash Flows (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	$ 1.8	$ 1.8
Pension
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	1.8	1.8
Expected contributions in next twelve months	1.8
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2020	20.6
2021	20.7
2022	20.9
2023	20.7
2024	20.3
2025-2029	100.6
OPEB
Defined Benefit Plan Disclosure [Line Items]
Employer contributions	5.8	$ 6.1
Expected contributions in next twelve months	7.0
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]
2020	7.0
2021	7.2
2022	7.3
2023	7.1
2024	7.1
2025-2029	$ 33.6

Amount of contribution received by defined benefit plan from employer which increases plan assets.

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