Postretirement Benefits - Reconciliation of Benefit Obligation and Plan Assets, Pension Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Changes in PBO:
PBO at beginning of year	$ 556,199
PBO at end of year	665,207	$ 556,199
Pension plan
Changes in PBO:
PBO at beginning of year	556,199	602,606
Service cost	19,929	21,092	$ 17,942
Interest cost	23,827	22,303	22,124
Plan amendments	0	2,377
Actuarial loss (gain)	88,053	(67,270)
Benefits paid	(22,801)	(24,909)
PBO at end of year	665,207	556,199	602,606
Changes in plan assets:
Fair value at beginning of year	392,672	377,624
Return on assets	77,290	(12,824)
Contributions	21,602	52,781
Benefits paid	(22,801)	(24,909)
Fair value at end of year	468,763	392,672	377,624
Funded status at year end	(196,444)	(163,527)
SERP
Changes in plan assets:
Contributions	$ 21,600	$ 52,800	$ 21,300