U.K. Pension Plan Major Categories of Plan Assets (Details) - Pension Plan [Member] - Foreign Plan [Member] - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Mar. 25, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 319,003	$ 270,169	$ 277,308
Growth pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	221,762	182,630
Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	30,494	29,463
Government bond pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47,679	47,030
Infrastructure pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,455	8,536
Long-term income pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8,485	2,510
Short duration index linked pooled fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 4,128	$ 0