Postretirement Benefits - Funding Policy/Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Postretirement benefits
Employer contributions during fiscal year	$ 1,571	$ 1,508
Net amount recognized in the Statements of Financial Position:
Funded status at end of year	(165,098)	(146,842)
Pension liabilities – due within one year	(752)	(1,183)
Pension liabilities – due after one year	(164,346)	(145,659)
Net amount recognized	$ (165,098)	$ (146,842)
Employee pension plan | Qualified Plan | Funded Plan
Postretirement benefits
Employer contributions during fiscal year			$ 80,000
Funding target (as a percentage of the funding target liability)	100.00%
Employee pension plan | Qualified Plan | Funded Plan | Minimum
Postretirement benefits
Minimum target normal cost threshold necessary for future pension funding	$ 80,000