Pension and Other Post-Retirement Benefit Plans (Fair Value of Plan Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Hedge funds and private equity [Member] | Measured at net asset value [Domain]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 403	$ 2,083
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	843,895	746,763	$ 625,646
Defined Benefit Plan Assets fair value hierarchy	843,492	744,680
Pension Plans [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	20,397	74,263
Pension Plans [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	823,095	670,417
Pension Plans [Member] | Measured at net asset value [Domain]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	403	2,083
Pension Plans [Member] | Common trust funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	5,055	3,467
Pension Plans [Member] | Common trust funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	0
Pension Plans [Member] | Common trust funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	5,055	3,467
Pension Plans [Member] | Corporate stock [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	20,353	14,995
Pension Plans [Member] | Corporate stock [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	19,852	14,553
Pension Plans [Member] | Corporate stock [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	501	442
Pension Plans [Member] | Mutual funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	237,063	176,777
Pension Plans [Member] | Mutual funds [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	0	59,710
Pension Plans [Member] | Mutual funds [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	237,063	117,067
Pension Plans [Member] | Fixed income investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	581,021	549,441
Pension Plans [Member] | Fixed income investments [Member] | Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	545	0
Pension Plans [Member] | Fixed income investments [Member] | Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan assets	$ 580,476	$ 549,441