Postretirement Benefits Other Than Pensions - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service costs, before tax	$ 3,800,000	$ 3,500,000
Unrecognized prior service costs, net of tax	2,300,000	2,100,000
Unrecognized actuarial losses, before tax	(128,700,000)	(86,800,000)
Unrecognized actuarial losses, net of tax	(79,300,000)	(53,100,000)
Prior service costs expected to be recognized next fiscal year	900,000
Prior service costs expected to be recognized next fiscal year, net of tax	600,000
Actuarial losses expected to be recognized next fiscal year	8,500,000
Actuarial losses expected to be recognized next fiscal year, net of tax	5,200,000
Accrued postretirement liability, current	2,600,000
Employer expected contribution in 2014	2,600,000
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Unrecognized prior service costs, before tax	677,500	742,000
Unrecognized prior service costs, net of tax	415,300	453,000
Unrecognized actuarial losses, before tax	(3,100,000)	(2,300,000)
Unrecognized actuarial losses, net of tax	(1,900,000)	(1,400,000)
Prior service costs expected to be recognized next fiscal year	91,800
Prior service costs expected to be recognized next fiscal year, net of tax	56,300
Actuarial losses expected to be recognized next fiscal year	62,600
Actuarial losses expected to be recognized next fiscal year, net of tax	38,400
Funded status at end of year	$ (39,126,000)	$ (37,230,000)