Schedule of Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,513	$ 3,092	$ 11,505	$ 7,131
Interest cost	8,213	5,624	16,575	12,971
Expected return on assets	(8,887)	(6,677)	(18,190)	(15,400)
Prior service cost (credit)	101	96	211	223
Actuarial loss (gain)	7,351	877	10,700	2,022
Special termination benefit	1,206		1,462
Net periodic benefit cost (credit)	13,497	3,012	22,263	6,947
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	34	51	68	93
Interest cost	229	305	464	559
Prior service cost (credit)	(570)	(887)	(1,150)	(1,627)
Actuarial loss (gain)	(77)	(73)	(154)	(133)
Net periodic benefit cost (credit)	$ (384)	$ (604)	$ (772)	$ (1,108)