Cost and Fair Value Basis of Long-Term Debt (details) (USD $) In Thousands	9 Months Ended
Sep. 30, 2011	Dec. 31, 2010	Sep. 30, 2010
Long Term Debt By Maturity Abstract
Debt Instrument, Description	On September 12, 2011, we issued $50 million of secured medium-term notes (MTNs) with an interest rate of 3.176 percent and a maturity date of September 15, 2021.
Long Term Debt Maturities Repayment Terms	Our long-term debt consists of secured MTNs with maturity dates from 2012 through 2035, interest rates ranging from 3.176 percent to 9.05 percent, and a weighted-average coupon rate of 5.93 percent. For the nine months ended September 30, 2011, we redeemed $10 million of MTNs.
Mediumterm Notes Abstract
Carrying Amount	$ 641,700	$ 601,700	$ 636,700
Estimated Fair Value	$ 774,186	$ 690,126	$ 740,731

Identification of the lender and information about a contractual promise to repay a short-term or long-term obligation, which includes borrowings under lines of credit, notes payable, commercial paper, bonds payable, debentures, and other contractual obligations for payment. This may include rationale for entering into the arrangement, significant terms of the arrangement, which may include amount, repayment terms, priority, collateral required, debt covenants, borrowing capacity, call features, participation rights, conversion provisions, sinking-fund requirements, voting rights, basis for conversion if convertible and remarketing provisions. The description may be provided for individual debt instruments, rational groupings of debt instruments, or by debt in total.

Disclosure of timing of required repayments, sinking fund requirements, and other redeemable securities at fixed or determinable prices and dates in the five years immediately following the date of the latest balance sheet presented in the financial statements, and the amount thereafter to fully repay the principal of long-term debt. These disclosures may be made either on an individual debt or security basis, by type of debt or security basis, or on a combined basis.

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