Retirement Benefit Plans (Summary of Net Periodic Benefit Costs) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 50	$ 45	$ 45
Interest cost on benefit obligations	7,314	8,441	8,008
Expected return on plan assets	(12,296)	(11,461)	(11,873)
Amortization of prior service cost	0	0	0
Recognized net loss	5,864	6,665	5,724
Net periodic benefit cost (benefit)	932	3,690	1,904
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost on benefit obligations	375	389	345
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	0	0
Recognized net loss	748	426	390
Net periodic benefit cost (benefit)	1,123	815	735
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	0	0	0
Interest cost on benefit obligations	92	125	123
Expected return on plan assets	0	0	0
Amortization of prior service cost	0	14	73
Recognized net loss	0	35	47
Net periodic benefit cost (benefit)	$ 92	$ 174	$ 243