Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,163	$ 3,769	$ 9,251	$ 11,612
Interest cost	4,279	4,056	12,481	12,470
Expected return on plan assets	(10,375)	(9,652)	(30,244)	(27,978)
Amortization of net loss and prior service costs	1,366	1,167	4,057	4,368
Net periodic benefit cost (reduction of cost)	(1,567)	(660)	(4,455)	472
Other Post-retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	17	91	112
Interest cost	161	155	459	514
Amortization of net loss and prior service costs	(95)	(128)	(338)	(224)
Net periodic benefit cost (reduction of cost)	$ 94	$ 44	$ 212	$ 402