Schedule I - Condensed Financial Information (Details 4) (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, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income	$ 2,211	$ (7,194)	$ 3,229	$ 3,289	$ 3,648	$ 2,316	$ 3,481	$ 5,063	$ 4,010	$ 3,083	$ 1,907	$ 3,050	$ 1,535	$ 14,508	$ 12,050
Derivative instruments
Unrealized losses on derivative instruments													(4,311)	(8,933)	(22,057)
Reclassification adjustment for losses included in net income													6,427	11,349	18,633
Net gain (loss) recognized in other comprehensive income													12,259	2,416	(3,424)
Tax benefit (expense) related to items of other comprehensive income (loss)													(4,598)	(838)	1,242
Other comprehensive income (loss)													7,661	1,578	(2,182)
Total comprehensive income													9,196	16,086	9,868
Parent company
CONDENSED STATEMENTS OF COMPREHENSIVE INCOME (LOSS)
Net income													1,535	14,508	12,050
Derivative instruments
Unrealized losses on derivative instruments													(1,039)	(2,145)	(6,254)
Reclassification adjustment for losses included in net income													1,901	3,356	5,781
Net gain (loss) recognized in other comprehensive income													862	1,211	(473)
Tax benefit (expense) related to items of other comprehensive income (loss)													(406)	(420)	171
Other comprehensive income (loss) from parent													456	791	(302)
Other comprehensive income (loss) from subsidiaries													7,205	787	(1,880)
Other comprehensive income (loss)													7,661	1,578	(2,182)
Total comprehensive income													$ 9,196	$ 16,086	$ 9,868