Pension and Other Postretirement Benefits - Amounts Recognized in Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Amounts recognized in the balance sheets:
Accrued benefit liability (noncurrent)	$ (122,627)	$ (105,924)
Pension Benefits [Member]
Amounts recognized in the balance sheets:
Prepaid benefit cost	5,689	5,797
Accrued benefit liability (current)	(3,628)	(3,878)
Accrued benefit liability (noncurrent)	(85,646)	(61,975)
Net funded status	(83,585)	(60,056)
Other Benefits [Member]
Amounts recognized in the balance sheets:
Prepaid benefit cost	0	0
Accrued benefit liability (current)	(2,188)	(2,665)
Accrued benefit liability (noncurrent)	(36,981)	(43,949)
Net funded status	$ (39,169)	$ (46,614)