Retirement Plans and Other Postretirement Benefits - Narrative (Details) - USD ($) $ in Thousands	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]
Employee hire date no longer eligible for noncontributory retirement plan	Dec. 08, 2009
Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions for next fiscal year		$ 80
Defined benefit plan, contributions by employer	$ 53
Defined benefit plan, benefits paid	53
Nonqualified Plan [Member] | Retirement Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected employer contributions for next fiscal year		$ 158
Defined benefit plan, contributions by employer	80
Defined benefit plan, benefits paid	$ 1,300