STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Comprehensive income:
Net income	$ 35,037	$ 58,247	$ 75,827	$ 99,901
Derivative instruments:
Reclassification of losses to net income, net of tax of $24 and $72 for each of the three and nine months ended Sept. 30, 2013 and 2012, respectively	44	43	129	129
Other comprehensive income	44	43	129	129
Comprehensive income	$ 35,081	$ 58,290	$ 75,956	$ 100,030