Employee Benefits - Additional Information (Detail) (Other Postretirement Benefit Plans [Member])	3 Months Ended
	Mar. 31, 2015
Other Postretirement Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Age of employees under condition one	55 years
Age of employees under condition two	50 years
Period of service under condition one	5 years
Period of service under condition two	15 years
Minimum period of service at the time of termination hired before 2001	15 years
Condition under retiree healthcare plan	We sponsor a retiree healthcare plan in which all employees age 55 with five years of service (or employees age 50 with 15 years of service who are terminated under conditions that entitle them to a severance benefit) are eligible to participate.