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Retirement Plans, Summary of Change in Plan Assets (Details) - USD ($)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Pension Plan [Member]    
Change in plan assets [Roll Forward]    
Fair value of plan assets at beginning of year $ 303,151,000 $ 294,995,000
Actual return on plan assets 63,939,000 (18,214,000)
Employer contribution 3,064,000 8,102,000
Gross benefits paid (11,957,000) (11,749,000)
Acquisition 0 30,017,000
Fair value of plan assets at end of year 358,197,000 303,151,000
SERP [Member]    
Change in plan assets [Roll Forward]    
Fair value of plan assets at beginning of year 0 0
Actual return on plan assets 0 0
Employer contribution 145,000 145,000
Gross benefits paid (145,000) (145,000)
Acquisition 0 0
Fair value of plan assets at end of year 0 0
Other Postretirement Benefits [Member]    
Change in plan assets [Roll Forward]    
Fair value of plan assets at beginning of year 0 0
Actual return on plan assets 0 0
Employer contribution 90,000 103,000
Gross benefits paid (90,000) (103,000)
Acquisition 0 0
Fair value of plan assets at end of year $ 0 $ 0