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Employee Benefit Plans - Annual returns on plan assets, expense (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Employee Benefit Plans          
Annual returns on plan assets (as percentage) 19.70% 25.80% (6.20%) 17.40% 8.20%
Defined benefit, expected expense in next fiscal year $ 1.1        
Defined benefit, expected expense current fiscal year $ 1.8