Summary of Fair Value of Plan Assets by Asset Category (Details) - USD ($) $ in Thousands		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		$ 36,476		$ 31,304		$ 32,374
Defined Benefit Plan, Sponsor Location [Extensible List]		us-gaap:ForeignPlanMember		us-gaap:ForeignPlanMember		us-gaap:ForeignPlanMember
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		$ 136		$ 236
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		36,340		31,068
Balanced Fund
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		26,816	[1]	23,317	[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		26,816	[1]	23,317	[2]
Fixed Income
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		9,524	[3]	4,512	[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		9,524	[3]	4,512	[4]
Growth Funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	[5]			3,239
Growth Funds | 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	[5]			3,239
Cash
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		136		236
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		$ 136		$ 236

[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 (38%), bonds (27%), other assets (31%) and cash (4%).

[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 (34%), bonds (36%), other assets (27%) and cash (3%).

[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 (92%) and cash (8%).

[4] 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 (93%), other assets (1%) and cash (6%).

[5] 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: equities (59%), bonds (16%), other assets (22%), and cash (3%).