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 Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 3,951	$ 3,316	$ 8,001	$ 6,771
Interest cost	7,142	7,419	14,291	14,849
Expected return on plan assets	(15,698)	(14,333)	(31,353)	(28,688)
Amortization of prior service cost	268	334	514	619
Amortization of actuarial losses (gains)	1,833	217	3,666	434
Net periodic benefit credit	(2,504)	(3,047)	(4,881)	(6,015)
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	136	126	279	264
Interest cost	349	417	709	825
Amortization of prior service cost	(736)	(714)	(1,472)	(1,428)
Amortization of actuarial losses (gains)	(63)	(86)	(127)	(171)
Net periodic benefit credit	$ (314)	$ (257)	$ (611)	$ (510)