CONDENSED STATEMENTS OF CASH FLOWS (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Statement of Cash Flows [Abstract]
Net Cash Provided by Operating Activities	$ 31,553	$ 55,766
Cash Flows from Investing Activities:
Capital Expenditures	(40,674)	(25,718)
Net Purchase of Restricted Investments in Margin Accounts	(463)	(862)
Investment in Long-Term Receivables	(2,053)	(1,947)
Proceeds from Long-Term Receivables	1,981	2,402
Net Cash Used in Investing Activities	(41,209)	(26,125)
Cash Flows from Financing Activities:
Net Repayments of Short-Term Credit Facilities	(17,800)	(31,500)
Proceeds from Issuance of Long-Term Debt	30,000	0
Payments for Issuance of Long-Term Debt	(210)	(11)
Net Cash Provided by (Used in) Financing Activities	11,990	(31,511)
Net Increase (Decrease) in Cash and Cash Equivalents	2,334	(1,870)
Cash and Cash Equivalents at Beginning of Period	2,020	2,678
Cash and Cash Equivalents at End of Period	$ 4,354	$ 808