Retirement Plans (Schedule of Costs of Retirement Plans) (Details) - Pension Plan - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Defined Contribution Plan Disclosure
Expenses associated with contributions made	$ 903	$ 561	$ 2,499	$ 1,749
United States
Defined Contribution Plan Disclosure
Expenses associated with contributions made	766	415	2,080	1,276
Canada
Defined Contribution Plan Disclosure
Expenses associated with contributions made	23	53	91	167
United Kingdom
Defined Contribution Plan Disclosure
Expenses associated with contributions made	$ 114	$ 93	$ 328	$ 306