Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,769	$ 3,611	$ 11,612	$ 10,487
Interest cost	4,056	4,022	12,470	13,033
Expected return on plan assets	(9,652)	(8,554)	(27,978)	(25,999)
Amortization of net loss and prior service costs	1,167	1,426	4,368	4,320
Net periodic benefit cost (reduction of cost)	(660)	505	472	1,841
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	17	45	112	119
Interest cost	155	204	514	613
Amortization of net loss and prior service costs	(128)	53	(224)	75
Net periodic benefit cost (reduction of cost)	$ 44	$ 302	$ 402	$ 807