Pension Plans and Other Postretirement Benefits - Data Relating to Funding Position of Defined Benefit Plans (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Fair value of plan assets at beginning of year	$ 1,503,614
Fair value of plan assets at end of year	1,623,635	$ 1,503,614
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	1,813,409	1,484,193
Service cost	24,372	20,520	$ 24,360
Interest cost	72,731	69,162	60,130
Amendments		4,619
Actuarial (gain) loss	(83,593)	300,444
Business combinations	247,340
Benefits paid	(69,728)	(65,529)
Benefit obligation at end of year	2,004,531	1,813,409	1,484,193
Fair value of plan assets at beginning of year	1,505,661	1,506,684
Actual return on plan assets	(14,069)	56,430
Employer contributions	8,367	8,076
Business combinations	194,903
Benefits and other payments	(69,728)	(65,529)
Fair value of plan assets at end of year	1,625,134	1,505,661	1,506,684
Funded status	(379,397)	(307,748)
Accrued liabilities recognized in the consolidated balance sheet	(379,397)	(307,748)
Net loss	494,279	512,473
Net prior service cost	277	(5,728)
Pre-tax adjustment to AOCI	494,556	506,745
Taxes	(194,608)	(198,897)
Net adjustment to AOCI	299,948	307,848
Other Postretirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Benefit obligation at beginning of year	67,502	60,592
Service cost	914	605	742
Interest cost	2,995	2,778	2,691
Plan participants' contributions	2,619	3,498
Actuarial (gain) loss	(2,431)	7,793
Business combinations	56,539
Medicare Part D reimbursement	420	495
Benefits paid	(7,061)	(8,259)
Benefit obligation at end of year	121,497	67,502	$ 60,592
Employer contributions	4,022	4,266
Plan participants' contributions	2,619	3,498
Medicare Part D reimbursement	420	495
Benefits and other payments	(7,061)	(8,259)
Funded status	(121,497)	(67,502)
Accrued liabilities recognized in the consolidated balance sheet	(121,497)	(67,502)
Net loss	4,200	6,737
Net prior service cost	(9,096)	(10,455)
Pre-tax adjustment to AOCI	(4,896)	(3,718)
Taxes	1,927	1,459
Net adjustment to AOCI	$ (2,969)	$ (2,259)