Summary of Fair Value of Plan Assets by Asset Category (Details) - UNITED KINGDOM - USD ($) $ in Thousands		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		$ 31,304		$ 32,374		$ 33,610
Estimate of Fair Value Measurement | Fair Value, Inputs, Level 1
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		236		180
ASU 2015-07 | Not Subject to Leveling
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	[1]	31,068		32,194
Mutual Funds | Balanced Fund
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		23,317	[2]	15,611	[3]
Mutual Funds | Balanced Fund | ASU 2015-07 | Not Subject to Leveling
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	[1]	23,317	[2]	15,611	[3]
Mutual Funds | Growth Funds
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		3,239	[4]	16,583	[5]
Mutual Funds | Growth Funds | ASU 2015-07 | Not Subject to Leveling
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	[1]	3,239	[4]	16,583	[5]
Mutual Funds | Fixed Income
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	[6]	4,512
Mutual Funds | Fixed Income | ASU 2015-07 | Not Subject to Leveling
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets	[1],[6]	4,512
Cash
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		236		180
Cash | Estimate of Fair Value Measurement | Fair Value, Inputs, Level 1
Defined Benefit Plans And Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair Value of Plan Assets		$ 236		$ 180

[1] In 2016, the Company adopted ASU 2015-07, “Fair Value Measurement (Topic 820): Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent).” As a result of the adoption, pension plan assets measured using the net asset value per share (or its equivalent) are presented as “Not Subject to Leveling” in the table above.

[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 a balanced return between capital growth and fixed income and is allocated on a weighted average basis as follows: equities (52%), bonds (24%), other assets (9%) and cash (15%).

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

[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 (67%), bonds (12%), other assets (9%), and cash (12%).

[6] This class comprises a diversified portfolio of global investments which seeks fixed income and is allocated on a weighted average basis as follows: bonds (93%), other assets (1%) and cash (6%).