Postretirement Benefits (Details 3) - USD ($) $ in Thousands				12 Months Ended
	Dec. 22, 2017	Aug. 15, 2017	Jan. 18, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Apr. 30, 2018	Jan. 31, 2018
Postretirement benefits
Increase in projected benefit obligations					$ 135,000
Employer contributions during fiscal year					61,102	$ 17,418
Funded status at end of year					(207,766)	(224,115)
Net amount recognized in the Statements of Financial Position:
Pension liabilities - due within one year					(236)	(2,288)
Pension liabilities - due after one year					(207,530)	(221,827)
Net amount recognized					$ (207,766)	$ (224,115)
Employee pension plan | Qualified plan | Funded plan
Postretirement benefits
Employer contributions during fiscal year	$ 19,900	$ 20,000	$ 19,000
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
Employee pension plan | Qualified plan | Funded plan | Scenario, Forecast
Postretirement benefits
Employer contributions during fiscal year				$ 80,000
Expected employer contributions during next fiscal year							$ 40,000	$ 40,000