Pension Plans and Other Postretirement Benefits - Data Relating to Funding Position of Defined Benefit Plans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Change in plan assets:
Fair value of plan assets at beginning of year	$ 1,640,300,000
Fair value of plan assets at end of year	2,013,117,000	$ 1,640,300,000
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,007,158,000	2,004,531,000
Service cost	20,193,000	25,037,000	$ 24,372,000
Interest cost	79,270,000	83,410,000	72,731,000
Amendments and curtailments		(28,308,000)
Actuarial (gain) loss	172,180,000	4,827,000
Benefits paid	(90,065,000)	(82,339,000)
Benefit obligation at end of year	2,188,736,000	2,007,158,000	2,004,531,000
Change in plan assets:
Fair value of plan assets at beginning of year	1,642,131,000	1,625,134,000
Actual return on plan assets	251,381,000	88,564,000
Employer contributions	211,444,000	10,772,000
Benefits paid	(90,065,000)	(82,339,000)
Fair value of plan assets at end of year	2,014,891,000	1,642,131,000	1,625,134,000
Funded status	(173,845,000)	(365,027,000)
Accrued liabilities recognized in the consolidated balance sheet	(173,845,000)	(365,027,000)
Net loss (gain)	460,622,000	460,562,000
Net prior service cost (credit)	2,948,000	3,505,000
Pre-tax adjustment to AOCI	463,570,000	464,067,000
Taxes	(121,873,000)	(182,611,000)
Net adjustment to AOCI	341,697,000	281,456,000
Other Postretirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	109,922,000	121,497,000
Service cost	1,172,000	1,595,000	914,000
Interest cost	3,716,000	4,971,000	2,995,000
Plan participants’ contributions	2,929,000	3,085,000
Amendments and curtailments	(30,088,000)
Actuarial (gain) loss	(8,511,000)	(10,553,000)
Medicare Part D reimbursement	630,000	592,000
Benefits paid	(11,133,000)	(11,265,000)
Benefit obligation at end of year	68,637,000	109,922,000	$ 121,497,000
Change in plan assets:
Employer contributions	7,574,000	7,588,000
Plan participants’ contributions	2,929,000	3,085,000
Medicare Part D reimbursement	630,000	592,000
Benefits paid	(11,133,000)	(11,265,000)
Funded status	(68,637,000)	(109,922,000)
Accrued liabilities recognized in the consolidated balance sheet	(68,637,000)	(109,922,000)
Net loss (gain)	(13,936,000)	(6,413,000)
Net prior service cost (credit)	(36,466,000)	(7,737,000)
Pre-tax adjustment to AOCI	(50,402,000)	(14,150,000)
Taxes	13,251,000	5,568,000
Net adjustment to AOCI	$ (37,151,000)	$ (8,582,000)