Benefit Obligations - Plans with Accumulated Benefit Obligation less than Plan Assets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Projected benefit obligation	$ 9,370	$ 7,782
Accumulated benefit obligation	9,295	7,666
Fair value of plan assets	11,233	9,982
Post-retirement Benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Accumulated benefit obligation	57,384	57,934
Fair value of plan assets	$ 71,712	$ 66,968