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, 2016 USD ($)
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	$ 1,623,635
Fair value of plan assets at end of year	1,640,300
Significant Unobservable Inputs (Level 3) [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	57,674
Fair value of plan assets at end of year	56,240
Other Private Real Estate [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	2,787
Purchase(Sales)	(1,111)
Total Realized/Unrealized Gains (Losses)	668
Fair value of plan assets at end of year	2,344
Other Private Equity [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	5,603
Fair value of plan assets at end of year	21,924
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	5,603
Purchase(Sales)	17,177
Total Realized/Unrealized Gains (Losses)	(856)
Fair value of plan assets at end of year	21,924
Other Hedge Funds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	119,549
Fair value of plan assets at end of year	106,250
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	37,688
Purchase(Sales)	(16,337)
Total Realized/Unrealized Gains (Losses)	(371)
Fair value of plan assets at end of year	20,980
Guaranteed Deposit Fund [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	11,596
Fair value of plan assets at end of year	10,992
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	11,596
Purchase(Sales)	(540)
Total Realized/Unrealized Gains (Losses)	(64)
Fair value of plan assets at end of year	10,992
Other Securities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	209,878
Fair value of plan assets at end of year	234,969
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	57,674
Purchase(Sales)	(811)
Total Realized/Unrealized Gains (Losses)	(623)
Fair value of plan assets at end of year	$ 56,240