Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,163	$ 4,093	$ 6,088	$ 7,843
Interest cost	4,279	4,291	8,202	8,414
Expected return on plan assets	(10,169)	(9,576)	(19,869)	(18,326)
Amortization of net loss and prior service costs	1,365	1,766	2,691	3,201
Net periodic benefit cost (reduction of cost)	(1,362)	574	(2,888)	1,132
Other Post-retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	28	47	63	95
Interest cost	161	179	298	359
Amortization of net loss and prior service costs	(94)	(48)	(243)	(96)
Net periodic benefit cost (reduction of cost)	$ 95	$ 178	$ 118	$ 358