Long-Term Debt and Financing Obligations (Notes)	6 Months Ended
Jun. 30, 2018
Debt Disclosure [Abstract]
Long-term Debt	Long-Term Debt and Financing Obligations $125 Million Senior Notes. On June 28, 2018, the Company entered into a Note Purchase Agreement (the "Agreement") with several institutional purchasers under which the Company issued and sold $125 million aggregate principal amount of 4.22% Senior Notes due August 15, 2028 (the "Notes"). The net proceeds from the issuance of these senior notes were used to repay outstanding short-term borrowings under the RCF for working capital and general corporate purposes. The Company will pay interest on the Notes semi-annually on February 15 and August 15 of each year until maturity, beginning on February 15, 2019. The Company may redeem the Notes, in whole or in part, at any time at a redemption price equal to 100% of the principal amount to be redeemed together with the interest on such principal amount accrued to the date of redemption, plus a make-whole amount based on the prevailing market interest rates. $65 Million RGRT Senior Notes. On June 28, 2018, the RGRT, a Texas grantor trust through which the Company finances its portion of nuclear fuel for Palo Verde, and the Company entered into a Note Purchase Agreement (the "RGRT Agreement") with several institutional purchasers. Under the terms of the RGRT Agreement, the RGRT issued and sold $65 million aggregate principal amount of 4.07% Senior Notes due August 15, 2025 (the "RGRT Notes"). The net proceeds from the RGRT Notes were used to repay outstanding short-term borrowings under the RCF to finance nuclear fuel purchases. The Company guaranteed the payment of principal and interest on the RGRT Notes. RGRT’s assets, liabilities and operations are consolidated in the Company’s financial statements and the RGRT Notes are included as long-term debt on the balance sheet. The RGRT will pay interest on the RGRT Notes semi-annually on February 15 and August 15 of each year until maturity, beginning on February 15, 2019. The RGRT may redeem the RGRT Notes, in whole or in part, at any time at a redemption price equal to 100% of the principal amount to be redeemed together with the interest on such principal amount accrued to the date of redemption, plus a make-whole amount based on the prevailing market interest rates. The issuance sale of both the Notes and the RGRT Notes was made in reliance on a private placement exemption from the registration provisions of the Securities Act of 1933, as amended.

Long-Term Debt and Financing Obligations

$125 Million Senior Notes. On June 28, 2018, the Company entered into a Note Purchase Agreement (the "Agreement") with several institutional purchasers under which the Company issued and sold $125 million aggregate principal amount of 4.22% Senior Notes due August 15, 2028 (the "Notes"). The net proceeds from the issuance of these senior notes were used to repay outstanding short-term borrowings under the RCF for working capital and general corporate purposes. The Company will pay interest on the Notes semi-annually on February 15 and August 15 of each year until maturity, beginning on February 15, 2019.

The Company may redeem the Notes, in whole or in part, at any time at a redemption price equal to 100% of the principal amount to be redeemed together with the interest on such principal amount accrued to the date of redemption, plus a make-whole amount based on the prevailing market interest rates.

$65 Million RGRT Senior Notes. On June 28, 2018, the RGRT, a Texas grantor trust through which the Company finances its portion of nuclear fuel for Palo Verde, and the Company entered into a Note Purchase Agreement (the "RGRT Agreement") with several institutional purchasers. Under the terms of the RGRT Agreement, the RGRT issued and sold $65 million aggregate principal amount of 4.07% Senior Notes due August 15, 2025 (the "RGRT Notes"). The net proceeds from the RGRT Notes were used to repay outstanding short-term borrowings under the RCF to finance nuclear fuel purchases. The Company guaranteed the payment of principal and interest on the RGRT Notes. RGRT’s assets, liabilities and operations are consolidated in the Company’s financial statements and the RGRT Notes are included as long-term debt on the balance sheet. The RGRT will pay interest on the RGRT Notes semi-annually on February 15 and August 15 of each year until maturity, beginning on February 15, 2019.

The RGRT may redeem the RGRT Notes, in whole or in part, at any time at a redemption price equal to 100% of the principal amount to be redeemed together with the interest on such principal amount accrued to the date of redemption, plus a make-whole amount based on the prevailing market interest rates.

The issuance sale of both the Notes and the RGRT Notes was made in reliance on a private placement exemption from the registration provisions of the Securities Act of 1933, as amended.