Benefit Plans - Contributions to Pension and Other Postretirement Benefit Plans (Details) - USD ($) $ in Millions	1 Months Ended
Dec. 31, 2016	Dec. 31, 2018
Pension Benefits \| Qualified Plan (ERP)
Benefit plans
Contributions made	$ 26.0
Expected contributions in 2019		$ 0.0
Pension Benefits \| Nonqualified Plan
Benefit plans
Expected contributions in 2019		0.2
Other Benefits \| Nonqualified Plan
Benefit plans
Expected contributions in 2019		$ 1.2

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

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Name:	us-gaap_DefinedBenefitPlanContributionsByEmployer
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration