Retirement Plans and Other Postretirement Benefits - Additional Information (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Retirement Plan And Other Postretirement Plans [Line Items]
Employee hire date no longer eligible for noncontributory retirement plan	Dec. 08, 2009
Defined Benefit Plan, qualified retirement plan, estimated employer contributions in current fiscal year	$ 2,000,000
Retirement Plans [Member]
Retirement Plan And Other Postretirement Plans [Line Items]
Defined Benefit Plan, non-qualified plans, estimated employer contributions for next fiscal year		40,000
Defined benefit plan, contributions by employer	20,000
Defined benefit plan, benefits paid	852,000
Assumed long-term rate of investment return	7.50%	8.00%
Other Post Retirement Benefits [Member]
Retirement Plan And Other Postretirement Plans [Line Items]
Expected employer contributions for next fiscal year		82,000
Defined benefit plan, contributions by employer	40,000
Defined benefit plan, benefits paid	$ 40,000