Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,750	$ 3,265
Interest cost	4,123	4,988
Expected return on plan assets	(8,750)	(8,891)
Amortization of net loss	1,435	1,468
Net periodic benefit cost	558	830
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	48	29
Interest cost	180	206
Amortization of net loss	(48)	(31)
Net periodic benefit cost	$ 180	$ 204