Benefit Obligations - Fair Value of Assets for Post-Retirement Benefit Plan by Asset Category (Detail) (Level 2 [Member], USD $) In Thousands, unless otherwise specified	Dec. 31, 2012		Dec. 31, 2011
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total	$ 71,712		$ 66,968
Commingled funds - domestic equities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total	14,431	[1]	13,302	[1]
Commingled funds - international equities [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total	14,292	[1]	13,536	[1]
Commingled funds - bonds [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Total	$ 42,989	[1]	$ 40,130	[1]

[1] The amounts consist of investments in funds not registered with U.S. Securities and Exchange Commission, with underlying investments primarily in the equity securities included in the S&P 500 Index, non-U.S. equity securities and investment grade fixed-income securities. The fair value was determined based on the net asset value per share of each investment at December 31, 2012 and 2011. These funds are classified as Level 2 in the fair value hierarchy as the Company could redeem these investments with the sponsoring investment management organizations at December 31, 2012 and 2011, and with at least monthly frequency on an ongoing basis.