Retirement Benefit Plans (Changes in Plan Assets and Funded Status of Pension Plans and Other Postretirement Benefit Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets:
Beginning balance	$ 192,734
Employer contributions	78
Ending balance	216,225	$ 192,734
Pension Plan [Member]
Change in benefit obligation:
Beginning balance	211,508	203,645
Service cost	50	45	$ 45
Interest cost	7,314	8,441	8,008
Actuarial loss (gain)	18,396	6,108
Benefits paid and administrative expenses	(7,950)	(6,731)
Ending balance (1)	229,318	211,508	203,645
Change in plan assets:
Beginning balance	192,922	161,369
Actual return on plan assets	31,253	18,284
Employer contributions	0	20,000
Benefits paid and administrative expenses	(7,950)	(6,731)
Ending balance	216,225	192,922	161,369
Funded status of the plan at year end (2)	(13,093)	(18,586)
SERP [Member]
Change in benefit obligation:
Beginning balance	11,806	10,518
Service cost	0	0	0
Interest cost	375	389	345
Actuarial loss (gain)	1,037	1,023
Benefits paid and administrative expenses	(122)	(124)
Ending balance (1)	13,096	11,806	10,518
Change in plan assets:
Beginning balance	0	0
Actual return on plan assets	0	0
Employer contributions	122	124
Benefits paid and administrative expenses	(122)	(124)
Ending balance	0	0	0
Funded status of the plan at year end (2)	(13,096)	(11,806)
Other Benefits [Member]
Change in benefit obligation:
Beginning balance	3,852	3,853
Service cost	0	0	0
Interest cost	92	125	123
Actuarial loss (gain)	(631)	59
Benefits paid and administrative expenses	(219)	(185)
Ending balance (1)	3,094	3,852	3,853
Change in plan assets:
Beginning balance	0	0
Actual return on plan assets	0	0
Employer contributions	219	185
Benefits paid and administrative expenses	(219)	(185)
Ending balance	0	0	$ 0
Funded status of the plan at year end (2)	$ (3,094)	$ (3,852)