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Long-Term Debt - Schedule of Carrying Amounts and Estimated Fair Values of Long-Term Debt (Detail) - USD ($)
$ in Thousands
Jun. 30, 2015
Dec. 31, 2014
Long-term debt    
Less: current maturities $ (20,050) $ (19,192)
Long-term debt, less current maturities 1,521,683 1,637,592
Carrying Amount [Member]    
Debt Instrument [Line Items]    
Total long-term debt 1,541,733 1,656,784
Long-term debt    
Less: current maturities (20,050) (19,192)
Long-term debt, less current maturities 1,521,683 1,637,592
Total long-term debt 1,541,733 1,656,784
Carrying Amount [Member] | Bonds [Member]    
Debt Instrument [Line Items]    
Unsecured debt 343,210 374,112
Carrying Amount [Member] | Centuri secured revolving credit and term loan facility [Member]    
Debt Instrument [Line Items]    
Centuri secured revolving credit and term loan facility 234,598 199,267
Carrying Amount [Member] | Centuri other debt obligations [Member]    
Debt Instrument [Line Items]    
Centuri other debt obligations 31,471 31,128
Carrying Amount [Member] | Debentures [Member]    
Debt Instrument [Line Items]    
Unamortized discount (5,046) (5,223)
Notes payable 902,454 902,277
Carrying Amount [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]    
Debt Instrument [Line Items]    
Notes payable 125,000 125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]    
Debt Instrument [Line Items]    
Notes payable 125,000 125,000
Carrying Amount [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]    
Debt Instrument [Line Items]    
Notes payable 250,000 250,000
Carrying Amount [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]    
Debt Instrument [Line Items]    
Notes payable 250,000 250,000
Carrying Amount [Member] | Debentures [Member] | 8% Series, due 2026 [Member]    
Debt Instrument [Line Items]    
Notes payable 75,000 75,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]    
Debt Instrument [Line Items]    
Notes payable 25,000 25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]    
Debt Instrument [Line Items]    
Notes payable 25,000 25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]    
Debt Instrument [Line Items]    
Notes payable 25,000 25,000
Carrying Amount [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]    
Debt Instrument [Line Items]    
Notes payable 7,500 7,500
Carrying Amount [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]    
Debt Instrument [Line Items]    
Line of credit facility 30,000 150,000
Carrying Amount [Member] | Variable-rate bonds [Member] | Tax-exempt Series A, due 2028 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 50,000 50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2003 Series A, due 2038 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 50,000 50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2008 Series A, due 2038 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 50,000 50,000
Carrying Amount [Member] | Variable-rate bonds [Member] | 2009 Series A, due 2039 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 50,000 50,000
Carrying Amount [Member] | Fixed-rate bonds [Member]    
Debt Instrument [Line Items]    
Unamortized discount (1,645) (1,943)
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.25% 2003 Series D, due 2038 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 20,000 20,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 5.00% 2004 Series B, due 2033 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 0 31,200
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.85% 2005 Series A, due 2035 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 100,000 100,000
Carrying Amount [Member] | Fixed-rate bonds [Member] | 4.75% 2006 Series A, due 2036 [Member]    
Debt Instrument [Line Items]    
Unsecured debt 24,855 24,855
Market Value [Member] | Centuri secured revolving credit and term loan facility [Member]    
Debt Instrument [Line Items]    
Centuri secured revolving credit and term loan facility 235,542 200,341
Market Value [Member] | Centuri other debt obligations [Member]    
Debt Instrument [Line Items]    
Centuri other debt obligations 31,749 31,127
Market Value [Member] | Debentures [Member] | Notes, 4.45%, due 2020 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 133,183 133,403
Market Value [Member] | Debentures [Member] | Notes, 6.1%, due 2041 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 146,568 157,290
Market Value [Member] | Debentures [Member] | Notes, 3.875%, due 2022 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 259,723 262,030
Market Value [Member] | Debentures [Member] | Notes, 4.875%, due 2043 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 256,505 280,903
Market Value [Member] | Debentures [Member] | 8% Series, due 2026 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 99,431 102,296
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.59% series, due 2017 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 27,063 27,573
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.78% series, due 2022 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 30,705 31,144
Market Value [Member] | Debentures [Member] | Medium-term notes, 7.92% series, due 2027 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 32,706 33,695
Market Value [Member] | Debentures [Member] | Medium-term notes, 6.76% series, due 2027 [Member]    
Debt Instrument [Line Items]    
Notes payable, fair value disclosure 8,896 9,156
Market Value [Member] | Debentures [Member] | Revolving credit facility and commercial paper [Member]    
Debt Instrument [Line Items]    
Line of credit facility, fair value 30,000 150,000
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 50,000
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 50,000
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 50,000
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 50,000
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 20,103 20,277
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 0 31,223
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 100,629 100,071
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 $ 25,005 $ 25,399