Postretirement Benefits - Funding Policy/Funded Status (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Postretirement benefits
Employer contributions during fiscal year	$ 1,508	$ 80,061
Net amount recognized in the Statements of Financial Position:
Funded status at end of year	(146,842)	(118,596)
Pension liabilities – due within one year	(1,183)	(1,730)
Pension liabilities – due after one year	(145,659)	(116,866)
Net amount recognized	$ (146,842)	(118,596)
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