Pension and Other Postretirement Benefits - Schedule of Net Periodic Benefit Cost and Weighted Average Assumptions (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]
Discount rate	4.25%	5.00%
Expected return on plan assets	8.00%	8.00%
Weighted-average rate of compensation increase	2.75%	3.00%
Qualified retirement plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 20,319	$ 17,725	$ 16,932
Interest cost	38,266	37,276	35,614
Expected return on plan assets	(45,780)	(40,114)	(36,538)
Amortization of net actuarial loss	23,883	14,348	10,478
Net periodic benefit cost	36,688	29,235	26,486
Qualified retirement plan [Member] \| Net benefit cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.00%	5.75%	6.00%
Expected return on plan assets	8.00%	8.00%	8.00%
Weighted-average rate of compensation increase	3.00%	3.25%	3.25%
SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	274	217	372
Interest cost	1,629	1,766	2,045
Amortization of net actuarial loss	683	631	1,155
Net periodic benefit cost	2,586	2,614	3,572
SERP [Member] \| Net benefit cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.00%	5.75%	6.00%
Expected return on plan assets	8.00%	8.00%	8.00%
Weighted-average rate of compensation increase	3.00%	3.25%	3.25%
PBOP [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	977	858	856
Interest cost	2,547	2,631	2,491
Expected return on plan assets	(2,404)	(2,379)	(2,093)
Amortization of transition obligation	867	867	867
Amortization of net actuarial loss	1,031	590	489
Net periodic benefit cost	$ 3,018	$ 2,567	$ 2,610
PBOP [Member] \| Net benefit cost [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	5.00%	5.75%	6.00%
Expected return on plan assets	8.00%	8.00%	8.00%
Weighted-average rate of compensation increase	3.00%	3.25%	3.25%

The amount of gains or losses recognized in net periodic benefit cost.

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- Definition

The amount of the transition obligation or asset recognized in net periodic benefit cost.

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- Definition

The interest rate used to adjust for the time value of money.

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- Definition

An assumption as to the rate of return on plan assets reflecting the average rate of earnings expected on the funds invested or to be invested to provide for the benefits included in the benefit obligation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (k)(3)

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-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 87

-Paragraph 264

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

-Paragraph 5

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 4: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 30

-Glossary Expected Long-Term Rate of Return on Plan Assets

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- Definition

Expected rate of compensation increases (for pay-related plans).

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- Definition

An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

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-Number 87

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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

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-Name Statement of Financial Accounting Standard (FAS)

-Number 132R

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Service Cost (Component of Net Periodic Pension Cost)

-URI http://asc.fasb.org/extlink&oid=6525008

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