Employee Benefit Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 18,543	$ 19,508	$ 23,366
Interest cost	55,873	59,719	70,797
Expected return on plan assets	(81,212)	(85,466)	(94,307)
Amortization of net transition obligation	0	0	65
Amortization of prior service cost / (credit)	910	809	745
Amortization of net actuarial gain (loss)	39,209	39,009	42,584
Effect of settlement loss	32,761	0	0
Other	0	0	49
Net periodic benefit cost (income)	66,084	33,579	43,299
Retiree Health and Life Insurance Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	313	309	711
Interest cost	463	482	766
Expected return on plan assets	(1,636)	(1,579)	(1,661)
Amortization of prior service cost / (credit)	(499)	(498)	(104)
Amortization of net actuarial gain (loss)	(759)	(667)	(673)
Net periodic benefit cost (income)	$ (2,118)	$ (1,953)	$ (961)