STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Comprehensive income:
Net income	$ 28,206	$ 30,294	$ 40,790	$ 41,654
Derivative instruments:
Reclassification of losses to net income, net of tax of $24 and $48 for each of the three and six months ended June 30, 2013 and 2012, respectively	43	43	85	86
Other comprehensive income	43	43	85	86
Comprehensive income	$ 28,249	$ 30,337	$ 40,875	$ 41,740