Pension Plans (Plan Assets for Qualified Pension Plan) (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		$ 765,182	$ 677,999
Total carrying value		$ 766,921	680,142
Liquidity		57.70%
Maximize return		42.30%
Qualified Pension Plan
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		$ 766,921	$ 680,142	$ 590,387
Expected return on plan assets		6.50%	7.50%
Managers’ securities holdings percentage maximum limit of total market value		7.50%
Quoted prices  in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		$ 187,663	$ 164,144
Total carrying value		187,680	164,211
Significant  other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, fair value plan assets before accrued income		215,231	193,923
Total carrying value		216,953	195,944
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		362,288	319,932
Total carrying value		362,288	319,987
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		26,951	17,597
Cash and cash equivalents | Quoted prices  in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		26,951	17,597
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
Cash and cash equivalents | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	0
Derivative
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		285
Derivative | Significant  other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		285
Futures contracts - liabilities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(15)	(1,210)
Futures contracts - liabilities | Significant  other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		(15)	(1,210)
Fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		214,961	195,133
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		214,961	195,133
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		130,490	102,423
Mutual funds | Quoted prices  in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		130,490	102,423
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]	362,288	319,932
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		362,288	319,932
Corporate stock
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		30,222	44,124
Corporate stock | Quoted prices  in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		30,222	44,124
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
Other Income
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		1,739	2,143
Other Income | Quoted prices  in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		17	67
Other Income | Significant  other observable inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		1,722	2,021
Other Income | Not Subject to Leveling [Member]
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	55
Assets | Futures contracts - liabilities
Defined Benefit Plan Disclosure [Line Items]
Total carrying value			0
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
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 | Quoted prices  in active markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Total carrying value		0	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.