Retirement Benefit Plans (Changes in Benefit Obligations and Plan Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation: at beginning of year	$ 479,281	$ 462,819
Service cost	15,381	14,098	$ 13,511
Interest cost	16,514	16,907	18,635
Plan participants' contributions
Plan amendments	(17,315)
Net actuarial (gain) loss	39,419	16,944
Benefits paid	(19,436)	(31,487)
Benefit obligation, end of year	513,844	479,281	462,819
Fair value of plan assets: at beginning of year	515,555	491,550
Actual return on plan assets	68,307	40,375
Employer contributions	1,132	16,123
Benefit payments	(19,436)	(31,487)
Expenses	(1,193)	(1,006)
Fair value of plan assets, end of year	564,365	515,555	491,550
Funded status at end of year-net asset (liability)				$ 50,521	$ 36,274
Unrecognized loss: at beginning of year	115,910	109,565
Net actuarial loss (gain)	(12,932)	6,345
Unrecognized loss at end of year	102,978	115,910	109,565
Projected benefit obligation	479,281	462,819	462,819	513,844	479,281
Accumulated benefit obligation				489,075	443,261
Fair value of plan assets	515,555	491,550	491,550	564,365	515,555
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation: at beginning of year	22,481	22,281
Service cost	129	170	117
Interest cost	668	773	891
Plan participants' contributions	636	1,269
Plan amendments		(1,224)
Net actuarial (gain) loss	993	1,844
Benefits paid	(1,871)	(2,632)
Benefit obligation, end of year	23,036	22,481	22,281
Fair value of plan assets: at beginning of year
Actual return on plan assets
Employer contributions	1,235	1,363
Benefit payments	(1,871)	(2,632)
Expenses
Fair value of plan assets, end of year
Funded status at end of year-net asset (liability)				(23,036)	(22,481)
Unrecognized loss: at beginning of year	(2,078)	(2,553)
Net actuarial loss (gain)	1,346	475
Unrecognized loss at end of year	(732)	(2,078)	(2,553)
Projected benefit obligation	22,481	22,281	22,281	23,036	22,481
Fair value of plan assets