Retirement Plans - Net Pension Expense (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Plans
Defined Benefit Plan Disclosure [Line Items]
Net pension expense	$ 21,823	$ 666	$ 23,780	$ 1,330