Retirement Plans - Benefit Obligations and Plan Expenses Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Employer contributions	$ 2,423,000	$ 0	$ 0
Estimated future benefit payments	0	165,976,000	28,330,000
Postretirement Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Employer contributions	460,000	377,000	392,000
Expected contribution	332,000
Estimated future benefit payments	4,806,000	$ 5,276,000	$ 3,320,000
Maximum | Pension Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block
Eligible contributions by employer	$ 10,000,000.0