Pension and Other Post-Retirement Benefit Plans (Funded Status) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ 3,447	$ 4,996
Change in benefit obligation [Roll Forward]
Obligation, beginning balance	779,031	690,729
Service cost	0	0	$ 0
Interest cost	27,880	31,299	29,135
Plan transfers	2,895	3,313
Defined Benefit Plan, Benefit Obligation, Benefits Paid	34,218	31,718
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	(2,435)	0
Actuarial (gain) loss	100,673	85,408
Obligation, ending balance	873,826	779,031	690,729
Change in plan assets [Roll Forward]
Plan assets, beginning balance	746,763	625,646
Actual gain on plan assets	120,057	144,108
Employer contributions	10,833	5,414
Plan assets, ending balance	843,895	746,763	625,646
Funded status
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	2,435	0
Post-Retirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	0	0
Change in benefit obligation [Roll Forward]
Obligation, beginning balance	3,707	3,824
Service cost	139	107	123
Interest cost	91	148	127
Plan transfers	0	0
Defined Benefit Plan, Benefit Obligation, Benefits Paid	362	365
Defined Benefit Plan, Benefit Obligation, Payment for Settlement	0	0
Actuarial (gain) loss	(74)	(7)
Obligation, ending balance	3,484	3,707	3,824
Change in plan assets [Roll Forward]
Plan assets, beginning balance	0	0
Actual gain on plan assets	0	0
Employer contributions	362	365
Plan assets, ending balance	0	0	$ 0
Funded status
Defined Benefit Plan, Accumulated Benefit Obligation, (Increase) Decrease for Settlement and Curtailment	(17)	0
Other Current Liabilities [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(1,348)	(3,796)
Other Current Liabilities [Member] | Post-Retirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(415)	(431)
Other Noncurrent Liabilities [Member] | Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	(32,030)	(33,468)
Other Noncurrent Liabilities [Member] | Post-Retirement Healthcare Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined Benefit Plan, Funded (Unfunded) Status of Plan	$ (3,069)	$ (3,276)