Pension Plans and Other Postretirement Benefits - Changes in Level 3 Pension Plan Assets Measured at Fair Value on Recurring Basis (Detail) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	$ 2,035,878
Fair value of plan assets at end of year	2,418,702
Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	232,237
Fair value of plan assets at end of year	283,754
Other Real Estate Partnerships [Member] | Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	17,966
Purchase(Sales)	3,581
Total Realized/Unrealized Gains (Losses)	1,684
Fair value of plan assets at end of year	23,231
Other Private Equity [Member] | Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	87,966
Purchase(Sales)	7,206
Total Realized/Unrealized Gains (Losses)	1,952
Fair value of plan assets at end of year	97,124
Other Hedge Funds [Member] | Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	115,778
Purchase(Sales)	30,000
Total Realized/Unrealized Gains (Losses)	7,123
Fair value of plan assets at end of year	152,901
Guaranteed Deposit Fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	10,527
Fair value of plan assets at end of year	10,498
Guaranteed Deposit Fund [Member] | Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	10,527
Total Realized/Unrealized Gains (Losses)	(29)
Fair value of plan assets at end of year	10,498
Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	439,185
Fair value of plan assets at end of year	479,393
Other Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	232,237
Purchase(Sales)	40,787
Total Realized/Unrealized Gains (Losses)	10,730
Fair value of plan assets at end of year	$ 283,754