Plan Assets Measured and Reported in Financial Statements at Fair Value (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 406,163	$ 338,081
Collective trust funds | US Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	148,435	101,747
Collective trust funds | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	147,756	101,956
Collective trust funds | International fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36,580	36,128
US government and US government agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35,944	51,897
US corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,514	19,346
International corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,455	5,887
Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,083	4,883
Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,396	16,237
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,422	4,883
Level 1 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,422	4,883
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	404,741	333,198
Level 2 | Collective trust funds | US Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	148,435	101,747
Level 2 | Collective trust funds | International equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	147,756	101,956
Level 2 | Collective trust funds | International fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	36,580	36,128
Level 2 | US government and US government agency securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	35,944	51,897
Level 2 | US corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	20,514	19,346
Level 2 | International corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	5,455	5,887
Level 2 | Mutual funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3,661
Level 2 | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 6,396	$ 16,237