Benefit Plans - Reconciliation of Amounts in Accumulated Other Comprehensive Loss That Have Not Yet Been Recognized as Components of Net Periodic Benefit Cost (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Actuarial gain (loss)	$ (13,015)
Net periodic benefit cost, beginning balance	(154,125)
Reclassification Adjustments, Net periodic benefit cost	11,647
Net periodic benefit cost, ending balance	(155,493)
Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss, beginning balance	(119,644)
Actuarial gain (loss)	(11,219)
Reclassification Adjustments, Actuarial loss	3,402
Actuarial loss, ending balance	(127,461)
Prior service (cost) credit, beginning balance	(101)
Reclassification Adjustments, Prior service (cost) credit	28
Prior service (cost) credit, ending balance	(73)
Postretirement Medical [Member]
Defined Benefit Plan Disclosure [Line Items]
Actuarial loss, beginning balance	(40,502)
Actuarial gain (loss)	(1,796)
Reclassification Adjustments, Actuarial loss	2,942
Actuarial loss, ending balance	(39,356)
Prior service (cost) credit, beginning balance	6,122
Reclassification Adjustments, Prior service (cost) credit	(2,982)
Prior service (cost) credit, ending balance	3,140
Postretirement Medical [Member] | Deep South and Somerset Exchange Business and the Florence and Laurel Distribution Business [Member]
Defined Benefit Plan Disclosure [Line Items]
Reclassification Adjustments, Recognized loss due to the divestiture	8,257
Recognized loss due to the divestiture, ending balance	$ 8,257