Pension Plans (Plan Assets for Qualified Pension Plan) (Details) - USD ($) $ in Thousands		Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		$ 677,999	$ 588,482
Defined benefit plan, fair value of plan assets, accrued income		2,143	1,905
Total carrying value		680,142	590,387
Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		164,144	151,500
Defined benefit plan, fair value of plan assets, accrued income		67	113
Total carrying value		164,211	151,613
Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		193,923	155,918
Defined benefit plan, fair value of plan assets, accrued income		2,021	1,740
Total carrying value		195,944	157,658
Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		0	0
Defined benefit plan, fair value of plan assets, accrued income		0	0
Total carrying value		0	0
Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		319,932	281,064
Defined benefit plan, fair value of plan assets, accrued income		55	52
Total carrying value		319,987	281,116
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		17,597	17,752
Cash and cash equivalents | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		17,597	17,752
Cash and cash equivalents | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Cash and cash equivalents | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Cash and cash equivalents | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		195,133	151,895
Fixed income securities | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Fixed income securities | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		195,133	151,895
Fixed income securities | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Fixed income securities | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		102,423	99,584
Mutual funds | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		102,423	99,584
Mutual funds | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Mutual funds | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Mutual funds | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Common trust funds
Defined Benefit Plan Disclosure [Line Items]
Total carrying value	[1]	319,932	281,064
Common trust funds | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Common trust funds | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Common trust funds | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Common trust funds | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		319,932	281,064
Corporate stock
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		44,124	34,164
Corporate stock | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		44,124	34,164
Corporate stock | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Corporate stock | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Corporate stock | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Assets | Futures contracts - liabilities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	4,023
Assets | Futures contracts - liabilities | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Assets | Futures contracts - liabilities | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	4,023
Assets | Futures contracts - liabilities | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Assets | Futures contracts - liabilities | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Liability | Futures contracts - liabilities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		1,210	0
Liability | Futures contracts - liabilities | Quoted prices in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Liability | Futures contracts - liabilities | Significant other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		1,210	0
Liability | Futures contracts - liabilities | Significant unobservable inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Liability | Futures contracts - liabilities | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		$ 0	$ 0

[1] 1) In accordance with ASU 2015-07, Fair Value Measurement (Topic 820), certain investments that are measured at fair value using the net asset value per share (or its equivalent) practical expedient have not been classified in the fair value hierarchy. The fair value amounts presented in this table are intended to permit reconciliation of the fair value hierarchy to the amounts presented for the total pension plan assets.