Pension Plans and Other Postretirement Benefits - Net Periodic Defined Benefit Cost for Defined Benefit Plans (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,137	$ 5,832	$ 12,519	$ 11,832
Interest cost on projected benefit obligation	20,822	17,732	41,705	35,507
Expected return on plan assets	(26,423)	(23,476)	(54,237)	(47,051)
Amortization of prior service credit	(789)	(1,478)	(1,614)	(3,003)
Amortization of net actuarial loss	6,773	11,237	15,073	22,412
Net periodic benefit cost	6,520	9,847	13,446	19,697
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	340	174	798	374
Interest cost on projected benefit obligation	1,281	652	2,486	1,302
Amortization of prior service credit	(330)	(329)	(680)	(679)
Amortization of net actuarial loss	30	28	30	53
Net periodic benefit cost	$ 1,321	$ 525	$ 2,634	$ 1,050