Retirement Plans (Schedule of Costs of Retirement Plans) (Details) - Pension Plan - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Defined Contribution Plan Disclosure
Expenses associated with contributions made	$ 902	$ 562	$ 1,596	$ 1,187
United States
Defined Contribution Plan Disclosure
Expenses associated with contributions made	770	410	1,314	861
Canada
Defined Contribution Plan Disclosure
Expenses associated with contributions made	34	54	68	113
United Kingdom
Defined Contribution Plan Disclosure
Expenses associated with contributions made	$ 98	$ 98	$ 214	$ 213