Retirement Benefit Plans (Summary of Components of Accumulated Other Comprehensive Loss Related to Pensions and Postretirement Benefits) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Deferred tax benefit		$ (2,632)	$ (2,502)	$ 918
Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)		59,433	65,857
Prior service cost		0	0
Total pre-tax amounts included in AOCL		59,433	65,857
Deferred tax benefit	$ 23,727	13,407
Amounts included in accumulated AOCL, net of tax		46,026	42,130
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)		3,299	3,009
Prior service cost		0	0
Total pre-tax amounts included in AOCL		3,299	3,009
Deferred tax benefit	1,084	744
Amounts included in accumulated AOCL, net of tax		2,555	1,925
Other Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss (gain)		(16)	616
Prior service cost		0	0
Total pre-tax amounts included in AOCL		(16)	616
Deferred tax benefit	$ 222	(3)
Amounts included in accumulated AOCL, net of tax		$ (13)	$ 394