U.K. Pension Plan Major Categories of Plan Assets (Details) - Pension Plan [Member] - Foreign Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 297,202	$ 319,003	$ 270,169
Fair Value Measured at Net Asset Value Per Share [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	297,202	319,003
Fair Value Measured at Net Asset Value Per Share [Member] | Growth pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	189,540	221,762
Fair Value Measured at Net Asset Value Per Share [Member] | Private equity pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31,715	30,494
Fair Value Measured at Net Asset Value Per Share [Member] | Government bond pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23,340	47,679
Fair Value Measured at Net Asset Value Per Share [Member] | Infrastructure pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	4,329	6,455
Fair Value Measured at Net Asset Value Per Share [Member] | Long-term income pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18,076	8,485
Fair Value Measured at Net Asset Value Per Share [Member] | Illiquid Growth Pooled Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,114	0
Fair Value Measured at Net Asset Value Per Share [Member] | Short duration index linked pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	23,102	4,128
Fair Value Measured at Net Asset Value Per Share [Member] | Other, net [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 986	$ 0