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, 2018 USD ($)
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	$ 2,013,117
Fair value of plan assets at end of year	1,831,773
Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	97,428
Fair value of plan assets at end of year	158,632
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	2,905
Purchase(Sales)	4,717
Total Realized/Unrealized Gains (Losses)	1,394
Fair value of plan assets at end of year	9,016
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	31,484
Purchase(Sales)	30,396
Total Realized/Unrealized Gains (Losses)	1,819
Fair value of plan assets at end of year	63,699
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	52,114
Purchase(Sales)	19,971
Total Realized/Unrealized Gains (Losses)	3,417
Fair value of plan assets at end of year	75,502
Guaranteed Deposit Fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	10,925
Fair value of plan assets at end of year	10,415
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	10,925
Total Realized/Unrealized Gains (Losses)	(510)
Fair value of plan assets at end of year	10,415
Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	304,463
Fair value of plan assets at end of year	361,178
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	97,428
Purchase(Sales)	55,084
Total Realized/Unrealized Gains (Losses)	6,120
Fair value of plan assets at end of year	$ 158,632