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Retirement Benefit Plans (Changes in Benefit Obligations and Plan Assets) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year $ 492,017 $ 513,844  
Service cost 10,981 12,414 $ 15,381
Interest cost 18,843 16,762 16,514
Net actuarial (gain) loss 81,166 (30,796)  
Benefits paid (21,141) (20,207)  
Benefit obligation, end of year 581,866 492,017 513,844
Fair value of plan assets at beginning of year 542,618 564,365  
Actual return on plan assets 130,745 (40,491)  
Employer contributions 101,165 40,138  
Benefit payments (21,141) (20,207)  
Expenses (1,249) (1,187)  
Fair value of plan assets, end of year 752,138 542,618 564,365
Funded status at end of year - net asset (liability) 170,272 50,601  
Unrecognized loss at beginning of year 149,470 102,978  
Net actuarial loss (gain) (13,218) 46,492  
Unrecognized gain (loss) at end of year 136,252 149,470 102,978
Accumulated benefit obligation 550,005 467,300  
Other Post-Retirement Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Benefit obligation at beginning of year 16,283 23,036  
Service cost 95 120 129
Interest cost 621 621 668
Plan participants' contributions 547 628  
Net actuarial (gain) loss 733 (6,717)  
Benefits paid (1,566) (1,405)  
Benefit obligation, end of year 16,713 16,283 23,036
Employer contributions 1,019 777  
Plan participants' contributions 547 628  
Benefit payments (1,566) (1,405)  
Funded status at end of year - net asset (liability) (16,713) (16,283)  
Unrecognized loss at beginning of year (7,015) (732)  
Net actuarial loss (gain) 1,646 (6,283)  
Unrecognized gain (loss) at end of year $ (5,369) $ (7,015) $ (732)