Pension Plans and Other Postretirement Benefits - Data Relating to Funding Position of Defined Benefit Plans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Change in plan assets:
Fair value of plan assets at beginning of year	$ 1,831,773,000
Employer contributions	0	$ 0	$ 200,000
Fair value of plan assets at end of year	2,035,878,000	1,831,773,000
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	1,949,613,000	2,188,736,000
Service cost	17,294,000	20,346,000	20,193,000
Interest cost	81,579,000	74,704,000	79,270,000
Actuarial (gain) loss	298,713,000	(228,897,000)
Benefits paid	(99,870,000)	(105,276,000)
Benefit obligation at end of year	2,247,329,000	1,949,613,000	2,188,736,000
Change in plan assets:
Fair value of plan assets at beginning of year	1,833,833,000	2,014,891,000
Actual return on plan assets	293,546,000	(90,657,000)
Employer contributions	10,431,000	14,875,000
Benefits paid	(99,870,000)	(105,276,000)
Fair value of plan assets at end of year	2,037,940,000	1,833,833,000	2,014,891,000
Funded status	(209,389,000)	(115,780,000)
Accrued liabilities recognized in the consolidated balance sheet	(209,389,000)	(115,780,000)
Net loss (gain)	507,029,000	401,716,000
Net prior service cost (credit)	1,834,000	2,391,000
Pre-tax adjustment to AOCI	508,863,000	404,107,000
Taxes	(133,779,000)	(106,240,000)
Net adjustment to AOCI	375,084,000	297,867,000
Other Postretirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	59,991,000	68,637,000
Service cost	859,000	938,000	1,172,000
Interest cost	2,344,000	2,293,000	3,716,000
Plan participants’ contributions	2,749,000	2,974,000
Actuarial (gain) loss	(687,000)	(4,758,000)
Medicare Part D reimbursement	370,000	508,000
Benefits paid	(9,134,000)	(10,601,000)
Benefit obligation at end of year	56,492,000	59,991,000	$ 68,637,000
Change in plan assets:
Employer contributions	6,015,000	7,119,000
Plan participants’ contributions	2,749,000	2,974,000
Medicare Part D reimbursement	370,000	508,000
Benefits paid	(9,134,000)	(10,601,000)
Funded status	(56,492,000)	(59,991,000)
Accrued liabilities recognized in the consolidated balance sheet	(56,492,000)	(59,991,000)
Net loss (gain)	(17,308,000)	(17,868,000)
Net prior service cost (credit)	(27,007,000)	(31,737,000)
Pre-tax adjustment to AOCI	(44,315,000)	(49,605,000)
Taxes	11,650,000	13,041,000
Net adjustment to AOCI	$ (32,665,000)	$ (36,564,000)