Retirement Plans and Other Postretirement Benefits - Additional Information (Detail) (USD $)	3 Months Ended	9 Months Ended		12 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Sep. 30, 2013	Sep. 30, 2013 Retirement Plans [Member]	Dec. 31, 2012 Retirement Plans [Member]	Sep. 30, 2013 Other Post Retirement Benefits [Member]	Dec. 31, 2012 Other Post Retirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Employee hire date no longer eligible for noncontributory retirement plan		Dec. 08, 2009
Defined Benefit Plan, qualified retirement plan, employer contributions in current fiscal year	$ 2,000,000	$ 2,000,000
Defined Benefit Plan, non-qualified plans, estimated employer contributions for next fiscal year				40,000
Expected employer contributions for next fiscal year						82,000
Defined benefit plan, non-qualified plans, contributions by employer			30,000
Defined benefit plan, contributions by employer					60,000
Defined benefit plan, benefits paid			$ 1,300,000		$ 60,000
Assumed long-term rate of investment return			7.50%	8.00%