Pension Plans and Other Postretirement Benefits - Data Relating to Funding Position of Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Change in plan assets:
Fair value of plan assets at beginning of year	$ 1,623,635
Fair value of plan assets at end of year	1,640,300	$ 1,623,635
Pension Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	2,004,531	1,813,409
Service cost	25,037	24,372	$ 20,520
Interest cost	83,410	72,731	69,162
Amendments and curtailments	(28,308)
Actuarial (gain) loss	4,827	(83,593)
Business combinations		247,340
Benefits paid	(82,339)	(69,728)
Benefit obligation at end of year	2,007,158	2,004,531	1,813,409
Change in plan assets:
Fair value of plan assets at beginning of year	1,625,134	1,505,661
Actual return on plan assets	88,564	(14,069)
Employer contributions	10,772	8,367
Business combinations		194,903
Benefits paid	(82,339)	(69,728)
Fair value of plan assets at end of year	1,642,131	1,625,134	1,505,661
Funded status	(365,027)	(379,397)
Accrued liabilities recognized in the consolidated balance sheet	(365,027)	(379,397)
Net loss (gain)	460,562	494,279
Net prior service cost (credit)	3,505	277
Pre-tax adjustment to AOCI	464,067	494,556
Taxes	(182,611)	(194,608)
Net adjustment to AOCI	281,456	299,948
Other Postretirement Benefits [Member]
Change in benefit obligation:
Benefit obligation at beginning of year	121,497	67,502
Service cost	1,595	914	605
Interest cost	4,971	2,995	2,778
Plan participants’ contributions	3,085	2,619
Actuarial (gain) loss	(10,553)	(2,431)
Business combinations		56,539
Medicare Part D reimbursement	592	420
Benefits paid	(11,265)	(7,061)
Benefit obligation at end of year	109,922	121,497	$ 67,502
Change in plan assets:
Employer contributions	7,588	4,022
Plan participants’ contributions	3,085	2,619
Medicare Part D reimbursement	592	420
Benefits paid	(11,265)	(7,061)
Funded status	(109,922)	(121,497)
Accrued liabilities recognized in the consolidated balance sheet	(109,922)	(121,497)
Net loss (gain)	(6,413)	4,200
Net prior service cost (credit)	(7,737)	(9,096)
Pre-tax adjustment to AOCI	(14,150)	(4,896)
Taxes	5,568	1,927
Net adjustment to AOCI	$ (8,582)	$ (2,969)