Pension and Other Postretirement - Components of Net Periodic Benefit Costs (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 5,453	$ 5,554	$ 5,423
Interest cost	10,757	9,310	10,510
Expected return on plan assets	(12,468)	(11,066)	(11,112)
Amortization of prior service credit	(48)	(54)	(55)
Curtailment gain	(359)	0	0
Net loss recognition	4,154	6,388	5,974
Net periodic benefit cost	7,489	10,132	10,740
Other Benefits [Member]
Components of net periodic benefit cost:
Service cost	49	125	116
Interest cost	1,647	1,910	2,077
Expected return on plan assets	0	0	0
Amortization of prior service credit	(100)	(108)	(111)
Curtailment gain	0	0
Net loss recognition	315	932	842
Net periodic benefit cost	$ 1,911	$ 2,859	$ 2,924