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, 2019 USD ($)
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	$ 1,831,773
Fair value of plan assets at end of year	2,035,878
Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	158,632
Fair value of plan assets at end of year	232,237
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	9,016
Purchase(Sales)	7,623
Total Realized/Unrealized Gains (Losses)	1,327
Fair value of plan assets at end of year	17,966
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	63,699
Purchase(Sales)	9,531
Total Realized/Unrealized Gains (Losses)	14,736
Fair value of plan assets at end of year	87,966
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	75,502
Purchase(Sales)	27,000
Total Realized/Unrealized Gains (Losses)	13,276
Fair value of plan assets at end of year	115,778
Guaranteed Deposit Fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	10,415
Fair value of plan assets at end of year	10,527
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	10,415
Total Realized/Unrealized Gains (Losses)	112
Fair value of plan assets at end of year	10,527
Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	361,178
Fair value of plan assets at end of year	439,185
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	158,632
Purchase(Sales)	44,154
Total Realized/Unrealized Gains (Losses)	29,451
Fair value of plan assets at end of year	$ 232,237