Postretirement Benefits Other Than Pensions - Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 4,729	$ 10,400	$ 12,969
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service and interest cost	2,487	2,039	1,939
Prior service cost	65	26	26
Unrecognized net loss	75	298	129
Other	98	2,286	1,868
Net periodic benefit cost	$ 2,725	$ 4,649	$ 3,962