Pension Plans and Other Postretirement Benefits - Data Relating to Funding Position of Defined Benefit Plans (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Change in plan assets:
Fair value of plan assets at beginning of year	$ 2,035,878,000
Employer contributions	300,000,000	$ 0
Fair value of plan assets at end of year	2,418,702,000	2,035,878,000
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,247,329,000	1,949,613,000
Service cost	19,944,000	17,294,000	$ 20,346,000
Interest cost	71,421,000	81,579,000	74,704,000
Actuarial (gain) loss	288,944,000	298,713,000
Benefits paid	(106,346,000)	(99,870,000)
Benefit obligation at end of year	2,521,292,000	2,247,329,000	1,949,613,000
Change in plan assets:
Fair value of plan assets at beginning of year	2,037,940,000	1,833,833,000
Actual return on plan assets	178,610,000	293,546,000
Employer contributions	310,378,000	10,431,000
Benefits paid	(106,346,000)	(99,870,000)
Fair value of plan assets at end of year	2,420,582,000	2,037,940,000	1,833,833,000
Funded status	(100,710,000)	(209,389,000)
Accrued liability recognized in the consolidated balance sheet	(165,380,000)	(209,389,000)
Net accrued liability recognized in the consolidated balance sheet	(100,710,000)	(209,389,000)
Net loss (gain)	684,780,000	507,029,000
Net prior service cost (credit)	1,277,000	1,834,000
Pre-tax adjustment to AOCI	686,057,000	508,863,000
Taxes	(178,375,000)	(133,779,000)
Net adjustment to AOCI	507,682,000	375,084,000
Prepaid asset recognized in the consolidated balance sheet	64,670,000
Other Postretirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	56,492,000	59,991,000
Service cost	970,000	859,000	938,000
Interest cost	1,741,000	2,344,000	2,293,000
Plan participants’ contributions	2,386,000	2,749,000
Actuarial (gain) loss	2,371,000	(687,000)
Medicare Part D reimbursement	574,000	370,000
Benefits paid	(9,253,000)	(9,134,000)
Benefit obligation at end of year	55,281,000	56,492,000	$ 59,991,000
Change in plan assets:
Employer contributions	6,293,000	6,015,000
Plan participants’ contributions	2,386,000	2,749,000
Medicare Part D reimbursement	574,000	370,000
Benefits paid	(9,253,000)	(9,134,000)
Funded status	(55,281,000)	(56,492,000)
Accrued liability recognized in the consolidated balance sheet	(55,281,000)	(56,492,000)
Net accrued liability recognized in the consolidated balance sheet	(55,281,000)	(56,492,000)
Net loss (gain)	(13,701,000)	(17,308,000)
Net prior service cost (credit)	(22,269,000)	(27,007,000)
Pre-tax adjustment to AOCI	(35,970,000)	(44,315,000)
Taxes	9,352,000	11,650,000
Net adjustment to AOCI	$ (26,618,000)	$ (32,665,000)