Retirement Plans and Other Postretirement Benefits - Amounts Recognized on the Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	$ 502,992	$ 90,570
Pension Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	165,861	0
Current liability	(15,700)	(14,578)
Noncurrent liability	(166,484)	(280,185)
Net amount recognized	(16,323)	(294,763)
Other Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	337,131	90,570
Current liability	0	0
Noncurrent liability	0	0
Net amount recognized	$ 337,131	$ 90,570