Retirement Benefit Plans (Changes in Benefit Obligations and Plan Assets) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation: at beginning of year	$ 462,819	$ 456,911
Service cost	14,098	13,511
Interest cost	16,907	18,635	$ 19,251
Plan participants' contributions
Plan amendments
Net actuarial (gain) loss	16,944	(8,154)
Benefits paid	(31,487)	(18,084)
Benefit obligation, end of year	479,281	462,819	456,911
Fair value of plan assets: at beginning of year	491,550	438,708
Actual return on plan assets	40,375	(14,421)
Employer contributions	16,123	86,123
Benefit payments	(31,487)	(18,084)
Expenses	(1,006)	(776)
Fair value of plan assets, end of year	515,555	491,550	438,708
Funded status at end of year-net asset (liability)				$ 36,274	$ 28,731
Unrecognized loss: at beginning of year	109,565	72,858
Net actuarial loss (gain)	6,345	36,707
Unrecognized loss at end of year	115,910	109,565	72,858
Projected benefit obligation	462,819	456,911	456,911	479,281	462,819
Accumulated benefit obligation				443,261	429,338
Fair value of plan assets	491,550	438,708	438,708	515,555	491,550
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation: at beginning of year	22,281	28,368
Service cost	170	117
Interest cost	773	891	1,140
Plan participants' contributions	1,269	1,334
Plan amendments	(1,224)
Net actuarial (gain) loss	1,844	(5,905)
Benefits paid	(2,632)	(2,524)
Benefit obligation, end of year	22,481	22,281	28,368
Fair value of plan assets: at beginning of year
Actual return on plan assets
Employer contributions	1,363	1,190
Benefit payments	(2,632)	(2,524)
Expenses
Fair value of plan assets, end of year
Funded status at end of year-net asset (liability)				(22,481)	(22,281)
Unrecognized loss: at beginning of year	(2,553)	3,358
Net actuarial loss (gain)	475	(5,911)
Unrecognized loss at end of year	(2,078)	(2,553)	3,358
Projected benefit obligation	22,281	28,368	28,368	22,481	22,281
Fair value of plan assets