STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Comprehensive income:
Net income	$ 58,247	$ 48,581	$ 99,901	$ 83,491
Derivative instruments:
Reclassification of losses to net income, net of tax of $24, $24, $72 and $72, respectively	43	42	129	128
Other comprehensive income	43	42	129	128
Comprehensive income	$ 58,290	$ 48,623	$ 100,030	$ 83,619