Summary of Fair Value of Plan Assets by Asset Category (Details) - USD ($) $ in Thousands	Dec. 31, 2018		Dec. 31, 2017		Dec. 31, 2016
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	$ 33,124		$ 36,476		$ 31,304
Estimate of Fair Value Measurement | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	109		136
Not Subject to Leveling | ASU 2015-07
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	33,015		36,340
Balanced Fund
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	24,564	[1]	26,816	[2]
Balanced Fund | Not Subject to Leveling | ASU 2015-07
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	24,564	[1]	26,816	[2]
Fixed Income
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	8,451	[3]	9,524	[4]
Fixed Income | Not Subject to Leveling | ASU 2015-07
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	8,451	[3]	9,524	[4]
Cash
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	109		136
Cash | Estimate of Fair Value Measurement | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	$ 109		$ 136

[1] This class comprises a diversified portfolio of global investments which seeks a balanced return between capital growth and fixed income and is allocated on a weighted average basis as follows: equities (42%), bonds (19%), other assets (38%) and cash (1%).

[2] This class comprises a diversified portfolio of global investments which seeks a balanced return between capital growth and fixed income and is allocated on a weighted average basis as follows: equities (38%), bonds (27%), other assets (31%) and cash (4%).

[3] This class comprises a diversified portfolio of global investments which seeks fixed income growth and is allocated on a weighted average basis as follows: bonds (86%) and cash (14%).

[4] This class comprises a diversified portfolio of global investments which seeks long-term capital growth and is allocated on a weighted average basis as follows: bonds (92%) and cash (8%).