Pension and Other Postretirement Benefits - Schedule of Funded Status and Reconciliation of Changes in Plans and Fair Value of Assets (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Pension asset	$ 102,962	$ 71,719
Noncurrent accrued pension costs	(27,862)	(21,664)
Current	(1,194)	(1,220)
Defined benefit pension plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning Balance	453,723	394,556
Service cost	4,593	3,419	3,218
Interest cost	17,664	19,151	20,161
Actuarial losses	30,145	64,391
Benefits paid	(30,289)	(30,561)
OPEB benefits extinguished by increased pension benefits	2,787	2,767
Ending Balance	478,623	453,723	394,556
Fair value at beginning of the year	503,778	548,518
Actual return (loss) on plan assets	80,234	(14,179)
Benefits paid	(30,289)	(30,561)
Fair value at end of the year	553,723	503,778	548,518
Funded status	75,100	50,055
Pension asset	102,962	71,719
Noncurrent accrued pension costs	(27,862)	(21,664)
Funded status	75,100	50,055
Prior service cost (credit)	6,432	7,664
Actuarial losses	287,091	306,835
Total recognized in accumulated other comprehensive loss (income)	293,523	314,499
Total	368,623	364,554
Accumulated benefit obligations ("ABO") of pension plans	468,258	446,019
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Beginning Balance	51,690	46,526
Service cost	154	118	106
Interest cost	2,036	2,288	2,489
Actuarial losses	4,425	6,664
Benefits paid	(1,284)	(1,139)
OPEB benefits extinguished by increased pension benefits	(2,787)	(2,767)
Ending Balance	54,234	51,690	46,526
Employer contributions	1,284	1,139
Benefits paid	(1,284)	(1,139)
Funded status	(54,234)	(51,690)
Current	(1,194)	(1,220)
Noncurrent	(53,040)	(50,470)
Funded status	(54,234)	(51,690)
Prior service cost (credit)	(84,164)	(100,335)
Actuarial losses	80,258	83,741
Total recognized in accumulated other comprehensive loss (income)	(3,906)	(16,594)
Total	$ (58,140)	$ (68,284)