Employees - Narrative (Details)	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2018 USD ($)
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan, Expected Future Benefit Payment, Total	$ 6,900,000	$ 6,900,000
Defined Benefit Plan, Expected Future Benefit Payment, Current Fiscal Quarter	300,000	300,000
Defined Benefit Plan, Expected Future Benefit Payment, Remainder of 2018	500,000	500,000
Defined Benefit Plan, Expected Future Benefit Payment, Year 2019	2,000,000.0	2,000,000.0
Defined Benefit Plan, Expected Future Benefit Payment, Year 2020	2,000,000	2,000,000
Defined Benefit Plan, Expected Future Benefit Payment, Year 2021	2,000,000	2,000,000
Defined Benefit Plan, Expected Future Benefit Payment, Year 2020	100,000	100,000
Contributions by employer	$ 20,000,000.0
Benefit plan settlements		$ 4,200,000