Retirement Plans (Components of Net Periodic Benefits Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 4,093	$ 3,611	$ 7,843	$ 6,876
Interest cost	4,291	4,023	8,414	9,011
Expected return on plan assets	(9,576)	(8,554)	(18,326)	(17,445)
Amortization of net loss	1,766	1,426	3,201	2,894
Net periodic benefit cost	574	506	1,132	1,336
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	47	45	95	74
Interest cost	179	203	359	409
Amortization of net loss	(48)	53	(96)	22
Net periodic benefit cost	$ 178	$ 301	$ 358	$ 505