Retirement Plans, Components of Net Periodic Benefit Cost, Pension Benefits (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Recognition of actuarial loss	[1]	$ (7,322)	$ (25,776)	$ 15,013
Recognition of prior service credit (cost)	[1]	(2,317)	(3,760)	(4,763)
Pension Plan [Member]
Components of net periodic benefit cost [Abstract]
Service cost		7,622	10,677	13,402
Interest cost		15,499	13,729	14,123
Expected return on plan assets		(22,406)	(18,195)	(16,805)
Actuarial loss (gain)		2,890	4,400	5,484
Prior service credit		0	0	0
Net periodic benefit cost		3,605	10,611	16,204
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		(7,396)	7,562	15,203
Recognition of actuarial loss		(2,890)	(37,833)	(5,484)
Recognition of prior service credit (cost)		0	0	0
Total recognized in other comprehensive income		(10,286)	(30,271)	9,719
Total recognized in net periodic benefit cost and other comprehensive income		(6,681)	(19,660)	25,923
SERP [Member]
Components of net periodic benefit cost [Abstract]
Service cost		0	0	0
Interest cost		49	58	65
Expected return on plan assets		0	0	0
Actuarial loss (gain)		23	28	26
Prior service credit		0	0	0
Net periodic benefit cost		72	86	91
Other changes in plan assets and benefit obligations recognized in other comprehensive income [Abstract]
Current year actuarial loss (gain)		(70)	42	12
Recognition of actuarial loss		(23)	(28)	(26)
Recognition of prior service credit (cost)		0	0	0
Total recognized in other comprehensive income		(93)	14	(14)
Total recognized in net periodic benefit cost and other comprehensive income		$ (21)	$ 100	$ 77

[1] Actuarial gains (losses) are amortized into other income (expense). (See Note 11 - Retirement Plans)