Fair values of Pension Plan Assets By Asset Category (Detail) (Pension Plans, Defined Benefit, USD $)	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 683,091,000		$ 636,648,000		$ 630,303,000
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	155,874,000		152,240,000
Commodity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	27,906,000		26,498,000
Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	393,002,000		347,516,000
Short Term Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	8,298,000		3,593,000
Venture Capital And Partnerships
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	98,011,000		106,801,000
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	12,801,000	[1]	4,477,000	[1]
Fair Value, Inputs, Level 1 | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 1 | Commodity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 1 | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	4,503,000	[1]	884,000	[1]
Fair Value, Inputs, Level 1 | Short Term Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	8,298,000	[1]	3,593,000	[1]
Fair Value, Inputs, Level 1 | Venture Capital And Partnerships
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	572,279,000	[1]	525,370,000	[1]
Fair Value, Inputs, Level 2 | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	155,874,000	[1]	152,240,000	[1]
Fair Value, Inputs, Level 2 | Commodity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	27,906,000	[1]	26,498,000	[1]
Fair Value, Inputs, Level 2 | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	388,499,000	[1]	346,632,000	[1]
Fair Value, Inputs, Level 2 | Short Term Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 2 | Venture Capital And Partnerships
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	98,011,000	[1]	106,801,000	[1]	96,552,000
Fair Value, Inputs, Level 3 | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]	308,000
Fair Value, Inputs, Level 3 | Commodity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 3 | Equity Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 3 | Short Term Funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	0	[1]	0	[1]
Fair Value, Inputs, Level 3 | Venture Capital And Partnerships
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension plan assets	$ 98,011,000	[1]	$ 106,801,000	[1]	$ 96,244,000

[1] See Note 1 under the caption Fair Value Measurements for a description of the fair value hierarchy.