Other Postretirement Benefit Plans - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2015
Compensation And Retirement Disclosure [Abstract]
Minimum year of service to qualify for post-retirement medical benefits	20 years
Minimum age to qualify for post-retirement medical benefits	60 years
Condition to qualify for post-retirement medical benefits	Employees who had worked 20 years and attained a minimum age of 60 by September 1, 2000