Employees - Narrative (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Contributions by employer		$ 0
Benefit plan settlements	$ 100,000	$ 4,200,000

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