Retirement Plans (Defined Contribution Plans) (Details) - Pension Plan $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) plan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Defined Contribution Plan Disclosure
Number of defined contribution plans | plan	6
Defined Contribution Plan, Cost | $	$ 3,314	$ 2,414	$ 3,154
CANADA
Defined Contribution Plan Disclosure
Number of defined contribution plans | plan	2
Defined Contribution Plan, Cost | $	$ 223	225	226
UNITED KINGDOM
Defined Contribution Plan Disclosure
Number of defined contribution plans | plan	2
Defined Contribution Plan, Cost | $	$ 450	376	494
UNITED STATES
Defined Contribution Plan Disclosure
Number of defined contribution plans | plan	2
Defined Contribution Plan, Cost | $	$ 2,641	$ 1,813	$ 2,434