Retirement Benefit Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 15,381	$ 14,098	$ 13,511
Interest cost		16,514	16,907	18,635
Expected return on plan assets		(37,632)	(34,554)	(32,833)
Amortization of net loss/ prior service cost		5,554	5,783	3,169
Net periodic benefit cost		(183)	2,234	2,482
Net (loss) gain recognized during the year		(5,554)	(5,783)	(3,169)
Net actuarial loss (gain)		(7,378)	12,128	39,876
Total recognized in other comprehensive income		(12,932)	6,345	36,707
Total recognized in net periodic benefit cost and other comprehensive income		$ (13,115)	$ 8,579	$ 39,189
Discount rate for benefit obligations		3.57%	4.10%	4.40%
Discount rate for net periodic benefit cost		4.10%	4.40%	4.11%
Expected long-term return on plan assets		7.25%	7.25%	7.50%
Rate of compensation increase	[1]
Pension Benefits [Member] | Graded Scale, At Age 20 [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase		7.00%	7.00%	7.00%
Pension Benefits [Member] | Graded Scale, At Age 60 [Member]
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase		2.00%	2.00%	2.00%
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost		$ 129	$ 170	$ 117
Interest cost		668	773	891
Amortization of net loss/ prior service cost		(353)	145	6
Net periodic benefit cost		444	1,088	1,014
Net (loss) gain recognized during the year		353	(145)	(6)
Net actuarial loss (gain)		993	620	(5,905)
Total recognized in other comprehensive income		1,346	475	(5,911)
Total recognized in net periodic benefit cost and other comprehensive income		$ 1,790	$ 1,563	$ (4,897)
Discount rate for benefit obligations		3.52%	3.95%	4.32%
Discount rate for net periodic benefit cost		3.95%	4.32%	4.02%

[1] Graded scale, declining from 7.00% at age 20 to 2.00% at age 60