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, 2017 USD ($)
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	$ 1,640,300
Fair value of plan assets at end of year	2,013,117
Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	56,240
Fair value of plan assets at end of year	97,428
Other Real Estate Partnerships [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	3,112
Fair value of plan assets at end of year	3,747
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	2,344
Purchase(Sales)	365
Total Realized/Unrealized Gains (Losses)	196
Fair value of plan assets at end of year	2,905
Other Private Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	21,924
Fair value of plan assets at end of year	31,484
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	21,924
Purchase(Sales)	6,821
Total Realized/Unrealized Gains (Losses)	2,739
Fair value of plan assets at end of year	31,484
Other Hedge Funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	106,250
Fair value of plan assets at end of year	178,080
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	20,980
Purchase(Sales)	29,839
Total Realized/Unrealized Gains (Losses)	1,295
Fair value of plan assets at end of year	52,114
Guaranteed Deposit Fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	10,992
Fair value of plan assets at end of year	10,925
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	10,992
Total Realized/Unrealized Gains (Losses)	(67)
Fair value of plan assets at end of year	10,925
Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	234,969
Fair value of plan assets at end of year	304,463
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	56,240
Purchase(Sales)	37,025
Total Realized/Unrealized Gains (Losses)	4,163
Fair value of plan assets at end of year	$ 97,428