CONSOLIDATED STATEMENTS OF COMPREHENSIVE INCOME (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Comprehensive income:
Net income	$ 7,131	$ 22,200	$ 5,742	$ 14,878	$ 7,912	$ 19,973	$ 8,478	$ 14,643	$ 49,951	$ 51,006	$ 42,749
Derivative instruments:
Reclassification of losses to net income, net of tax of $51, $51 and $51, respectively									77	76	76
Other comprehensive income									77	76	76
Comprehensive income									$ 50,028	$ 51,082	$ 42,825