Pension Plans and Other Postretirement Benefits - Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total	$ 25,843	$ 25,446
Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Total	12,165	12,402
Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Total	588	716
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total	588	716
Level 1 | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
Level 1 | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Total	588	716
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total	25,255	24,730
Level 2 | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Total	12,165	12,402
Level 2 | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
Level 3 | Fixed income funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
Level 3 | Short-term investment funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
United States | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	10,517	9,836
United States | Level 1 | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
United States | Level 2 | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	10,517	9,836
United States | Level 3 | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
International | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	2,573	2,492
International | Level 1 | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	0	0
International | Level 2 | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	2,573	2,492
International | Level 3 | Equity security funds
Defined Benefit Plan Disclosure [Line Items]
Total	$ 0	$ 0