STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Comprehensive income:
Net income	$ 106,369	$ 89,901	$ 78,067
Derivative instruments:
Reclassification of losses to net income, net of tax of $97, $98 and $96, respectively	172	171	172
Total derivative instruments, net of tax	172	171	172
Other comprehensive income	172	171	172
Comprehensive income	$ 106,541	$ 90,072	$ 78,239