Postretirement Benefits Other Than Pensions - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014 Y	Dec. 31, 2013	Dec. 31, 2001 Y
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Minimum age of retirement under option one to avail benefit of post retirement	55
Minimum service period served at the time of retirement under option one to avail benefit of post retirement	15 years
Minimum age of retirement under option two to avail benefit of post retirement	65
Minimum service period served at time of retirement under option two to avail benefit of post retirement	10 years
Amendments		$ (2,800,000)
Minimum for age plus years of service of retirement under amended plan to avail benefit of post retirement			60
Minimum service period served at the time of retirement under amended plan to avail benefit of post retirement			10 years
Other Postretirement Benefit Plan [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amendments		(2,828,000)
Expected contribution to plan in 2014	600,000
Net loss recognized in other comprehensive income	900,000	300,000
Amortization of net loss	400,000	500,000
Amortization of prior service credit recognized in other comprehensive income	1,300,000	1,300,000
Unrecognized actuarial losses	6,800,000	6,300,000
Unrecognized actuarial losses, net of tax	4,100,000	3,900,000
Unrecognized Prior service credit	16,400,000	17,700,000
Unrecognized Prior service credit, Net of Tax	9,900,000	10,800,000
Net loss that will be amortized from accumulated other comprehensive loss into net post retirement benefit expense	500,000
Prior service credit in OCI that will be amortized into net post retirement benefit expense in next fiscal year	1,300,000