Retirement Plans, Fair Value of Pension Plan Investments Table (Details) - RAP - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan Investments
RAP Investment Amount	$ 478,849	$ 442,034	$ 390,564
Fair Value, Inputs, Level 1
Pension Plan Investments
RAP Investment Amount	427,983	391,979
Fair Value, Inputs, Level 2
Pension Plan Investments
RAP Investment Amount	0	0
Fair Value, Inputs, Level 3
Pension Plan Investments
RAP Investment Amount	50,866	50,055
Money market account
Pension Plan Investments
RAP Investment Amount	9,429	8,903
Money market account | Fair Value, Inputs, Level 1
Pension Plan Investments
RAP Investment Amount	9,429	8,903
Money market account | Fair Value, Inputs, Level 2
Pension Plan Investments
RAP Investment Amount	0	0
Money market account | Fair Value, Inputs, Level 3
Pension Plan Investments
RAP Investment Amount	0	0
Common /collective trust funds
Pension Plan Investments
RAP Investment Amount	172,950	155,964
Common /collective trust funds | Fair Value, Inputs, Level 1
Pension Plan Investments
RAP Investment Amount	172,950	155,964
Common /collective trust funds | Fair Value, Inputs, Level 2
Pension Plan Investments
RAP Investment Amount	0	0
Common /collective trust funds | Fair Value, Inputs, Level 3
Pension Plan Investments
RAP Investment Amount	0	0
Mutual funds
Pension Plan Investments
RAP Investment Amount	245,605	227,112
Mutual funds | Fair Value, Inputs, Level 1
Pension Plan Investments
RAP Investment Amount	245,605	227,112
Mutual funds | Fair Value, Inputs, Level 2
Pension Plan Investments
RAP Investment Amount	0	0
Mutual funds | Fair Value, Inputs, Level 3
Pension Plan Investments
RAP Investment Amount	0	0
Group annuity contracts
Pension Plan Investments
RAP Investment Amount	50,866	50,055
Group annuity contracts | Fair Value, Inputs, Level 1
Pension Plan Investments
RAP Investment Amount	0	0
Group annuity contracts | Fair Value, Inputs, Level 2
Pension Plan Investments
RAP Investment Amount	0	0
Group annuity contracts | Fair Value, Inputs, Level 3
Pension Plan Investments
RAP Investment Amount	$ 50,866	$ 50,055