STATEMENTS OF COMPREHENSIVE INCOME (UNAUDITED) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Comprehensive income:
Net income	$ 12,584	$ 11,360
Derivative instruments:
Reclassification of losses to net income, net of tax of $24 for each of the three months ended March 31, 2013 and 2012	42	43
Other comprehensive income	42	43
Comprehensive income	$ 12,626	$ 11,403