CONDENSED STATEMENTS OF CASH FLOWS (UNAUDITED) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012
Statement of Cash Flows [Abstract]
Net Cash Provided by Operating Activities	$ 84,960	$ 47,184
Cash Flows from Investing Activities:
Capital Expenditures	(76,138)	(77,548)
Net (Purchase) Sale of Restricted Investments in Margin Accounts	(1,060)	458
Investment in Long-Term Receivables	(3,256)	(2,871)
Proceeds from Long-Term Receivables	3,433	3,781
Net Cash Used in Investing Activities	(77,021)	(76,180)
Cash Flows from Financing Activities:
Net (Repayment of) Borrowing from Short-Term Credit Facilities	(9,100)	26,900
Proceeds from Issuance of Long-Term Debt	0	35,000
Principal Repayments of Long-Term Debt	0	(35,000)
Premium for Early Retirement of Debt	0	(700)
Payments for Issuance of Long-Term Debt	(5)	(341)
Net Cash (Used in) Provided by Financing Activities	(9,105)	25,859
Net Decrease in Cash and Cash Equivalents	(1,166)	(3,137)
Cash and Cash Equivalents at Beginning of Period	2,678	3,504
Cash and Cash Equivalents at End of Period	$ 1,512	$ 367