Postretirement Benefits - Reconciliation of Benefit Obligation and Plan Assets, Pension Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Changes in PBO:
PBO at beginning of year	$ 665,207
PBO at end of year	763,313	$ 665,207
Pension plan
Changes in PBO:
PBO at beginning of year	665,207	556,199
Service cost	24,461	19,929	$ 21,092
Interest cost	22,825	23,827	22,303
Actuarial loss (gain)	74,006	88,053
Benefits paid	(23,186)	(22,801)
PBO at end of year	763,313	665,207	556,199
Changes in plan assets:
Fair value at beginning of year	468,763	392,672
Return on assets	62,104	77,290
Contributions	21,851	21,602
Benefits paid	(23,186)	(22,801)
Fair value at end of year	529,532	468,763	$ 392,672
Funded status at year end	$ (233,781)	$ (196,444)