Postretirement Benefits - Funding Policy/Funded Status (Details) - USD ($) $ in Thousands			12 Months Ended
	Dec. 22, 2017	Jan. 18, 2017	Dec. 31, 2018	Dec. 31, 2017
Postretirement benefits
Employer contributions during fiscal year			$ 80,061	$ 61,102
Net amount recognized in the Statements of Financial Position:
Funded status at end of year			(118,596)	(207,766)
Pension liabilities - due within one year			(1,730)	(236)
Pension liabilities - due after one year			(116,866)	(207,530)
Net amount recognized			(118,596)	$ (207,766)
Employee pension plan | Qualified Plan | Funded Plan
Postretirement benefits
Employer contributions during fiscal year	$ 39,900	$ 19,000	$ 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