Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) (USD $)	Mar. 31, 2015	Dec. 31, 2014
Long-term debt
Less: current maturities	$ (18,266,000)us-gaap_LongTermDebtCurrent	$ (19,192,000)us-gaap_LongTermDebtCurrent
Long-term debt, less current maturities	1,506,527,000us-gaap_LongTermDebtNoncurrent	1,637,592,000us-gaap_LongTermDebtNoncurrent
Carrying Amount [Member]
Debt Instrument [Line Items]
Total long-term debt	1,524,793,000	1,656,784,000
Long-term debt
Less: current maturities	(18,266,000)	(19,192,000)
Long-term debt, less current maturities	1,506,527,000	1,637,592,000
Total long-term debt	1,524,793,000	1,656,784,000
Carrying Amount [Member] \| Bonds [Member]
Debt Instrument [Line Items]
Unsecured debt	374,135,000	374,112,000
Carrying Amount [Member] \| Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	219,995,000	199,267,000
Carrying Amount [Member] \| Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	28,298,000	31,128,000
Carrying Amount [Member] \| Debentures [Member]
Debt Instrument [Line Items]
Unamortized discount	(5,135,000)	(5,223,000)
Notes payable	902,365,000	902,277,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable	125,000,000	125,000,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable	125,000,000	125,000,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	250,000,000	250,000,000
Carrying Amount [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable	250,000,000	250,000,000
Carrying Amount [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable	75,000,000	75,000,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	25,000,000	25,000,000
Carrying Amount [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable	7,500,000	7,500,000
Carrying Amount [Member] \| Debentures [Member] \| Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility		150,000,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Unsecured debt	50,000,000	50,000,000
Carrying Amount [Member] \| Fixed-rate bonds [Member]
Debt Instrument [Line Items]
Unamortized discount	(1,920,000)	(1,943,000)
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Unsecured debt	20,000,000	20,000,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Unsecured debt	31,200,000	31,200,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Unsecured debt	100,000,000	100,000,000
Carrying Amount [Member] \| Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Unsecured debt	24,855,000	24,855,000
Market Value [Member] \| Centuri secured revolving credit and term loan facility [Member]
Debt Instrument [Line Items]
Centuri secured revolving credit and term loan facility	220,196,000	200,341,000
Market Value [Member] \| Centuri other debt obligations [Member]
Debt Instrument [Line Items]
Centuri other debt obligations	28,480,000	31,127,000
Market Value [Member] \| Debentures [Member] \| Notes, 4.45%, due 2020 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	135,578,000	133,403,000
Market Value [Member] \| Debentures [Member] \| Notes, 6.1%, due 2041 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	161,798,000	157,290,000
Market Value [Member] \| Debentures [Member] \| Notes, 3.875%, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	267,618,000	262,030,000
Market Value [Member] \| Debentures [Member] \| Notes, 4.875%, due 2043 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	290,295,000	280,903,000
Market Value [Member] \| Debentures [Member] \| 8% Series, due 2026 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	103,938,000	102,296,000
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.59% series, due 2017 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	27,398,000	27,573,000
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.78% series, due 2022 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	31,575,000	31,144,000
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 7.92% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	34,418,000	33,695,000
Market Value [Member] \| Debentures [Member] \| Medium-term notes, 6.76% series, due 2027 [Member]
Debt Instrument [Line Items]
Notes payable, fair value disclosure	9,375,000	9,156,000
Market Value [Member] \| Debentures [Member] \| Revolving credit facility and commercial paper [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value		150,000,000
Market Value [Member] \| Variable-rate bonds [Member] \| Tax-exempt Series A, due 2028 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2003 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2008 Series A, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
Market Value [Member] \| Variable-rate bonds [Member] \| 2009 Series A, due 2039 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	50,000,000	50,000,000
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.25% 2003 Series D, due 2038 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	20,216,000	20,277,000
Market Value [Member] \| Fixed-rate bonds [Member] \| 5.00% 2004 Series B, due 2033 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	31,329,000	31,223,000
Market Value [Member] \| Fixed-rate bonds [Member] \| 4.85% 2005 Series A, due 2035 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	100,651,000	100,071,000
Market Value [Member] \| Fixed-rate bonds [Member] \| 4.75% 2006 Series A, due 2036 [Member]
Debt Instrument [Line Items]
Line of credit facility, fair value	$ 25,406,000	$ 25,399,000

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The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

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- Definition

The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Balance Type:	credit
Period Type:	instant

- Definition

Amount of long-term debt, after unamortized discount or premium, scheduled to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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Balance Type:	credit
Period Type:	instant

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- Definition

The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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No definition available.

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- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, excluding amounts to be repaid within one year or the normal operating cycle, if longer (current maturities). Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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- Definition

Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer.

+ References

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- Definition

Fair value portion of notes payable.

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- Definition

Including both current and noncurrent portions, carrying amount as of the balance sheet date of other forms of debt not elsewhere specified in the taxonomy with initial maturities beyond one year or beyond the normal operating cycle, if longer.

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- Definition

Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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- Definition

Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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