Total Investments Plans Measured 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	$ 644,931	$ 601,448
Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	516,428	482,767
Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	116,198	102,785
Cash and Accrued Items
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12,305	15,896
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	122,032	522,825
Level 1 | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	82,390	482,767
Level 1 | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	27,337	24,162
Level 1 | Cash and Accrued Items
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12,305	15,896
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	521,323	78,623
Level 2 | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	432,462
Level 2 | Equity Securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	88,861	78,623
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,576
Level 3 | Fixed Income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,576