Retirement Plans, Components of Net Periodic Benefit Cost, Pension Benefits (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Recognition of actuarial loss	[1]	$ (25,776)	$ 15,013	$ (19,482)
Recognition of prior service credit (cost)	[1]	(3,760)	(4,763)	(6,963)
Pension Plan [Member]
Components of net periodic benefit cost [Abstract]
Service cost		10,677	13,402	14,683
Interest cost		13,729	14,123	13,302
Expected return on plan assets		(18,195)	(16,805)	(17,921)
Actuarial loss (gain)		4,400	5,484	7,728
Prior service credit		0	0	0
Net periodic benefit cost		10,611	16,204	17,792
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		7,562	15,203	(19,127)
Recognition of actuarial loss		(37,833)	(5,484)	(7,728)
Recognition of prior service credit (cost)		0	0	0
Total recognized in other comprehensive income		(30,271)	9,719	(26,855)
Total recognized in net periodic benefit cost and other comprehensive income		(19,660)	25,923	(9,063)
SERP [Member]
Components of net periodic benefit cost [Abstract]
Service cost		0	0	0
Interest cost		58	65	64
Expected return on plan assets		0	0	0
Actuarial loss (gain)		28	26	28
Prior service credit		0	0	0
Net periodic benefit cost		86	91	92
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		42	12	(30)
Recognition of actuarial loss		(28)	(26)	(28)
Recognition of prior service credit (cost)		0	0	0
Total recognized in other comprehensive income		14	(14)	(58)
Total recognized in net periodic benefit cost and other comprehensive income		$ 100	$ 77	$ 34

[1] Actuarial gains (losses) are amortized into costs of sales and operating expenses or selling, general and administrative expenses as appropriate. (See Note 10 - Retirement Plans).