Pension and Other Postretirement Benefits - Fair Values of Pension Plan Assets by Asset Category (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	$ 198,156	$ 182,370
Large-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	75,165	65,495
Mid-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	13,046	11,419
Small-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	18,785	17,184
Government Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	28,429	26,151
Corporate Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	24,421	20,971
Fixed Income Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	38,072	40,958
Cash & Equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	238	192
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	238	192
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Large-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Mid-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Small-Cap Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Government Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Bond Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Cash & Equivalents [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	238	192
Significant Observable Inputs (Level 2) [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	38,072	40,958
Significant Observable Inputs (Level 2) [Member] | Fixed Income Fund [Member]
Pension And Other Employee Benefit Plans [Line Items]
Fair values of pension plan assets by asset category	$ 38,072	$ 40,958