Statement of Owner's Equity (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Statement Of Owner's Equity Abstract
Owner's capital - balance at beginning of period	$ 1,046,862	$ 1,027,862	$ 978,682
Capital contribution from parent	5,100	19,000	49,180
Owner's capital - balance at end of period	1,051,962	1,046,862	1,027,862
Loans (to) from affiliate - balance at beginning of period	0	(105,431)	(34,265)
Loans (to) from affiliate	0	105,431	(71,166)
Loans (to) from affiliate - balance at end of period	0	0	(105,431)
Retained earnings - balance at beginning of period	242,396	284,319	265,668
Net income	152,813	149,527	153,651
Cash distributions	(127,000)	(191,450)	(135,000)
Retained earnings - balance at end of period	268,209	242,396	284,319
Accumulated other comprehensive income (loss) - balance at beginning of period	338	400	462
Cash flow hedges: Reclassification of gain into earnings	(62)	(62)	(62)
Accumulated other comprehensive income (loss) - balance at end of period	276	338	400
Total owner's equity	$ 1,320,447	$ 1,289,596	$ 1,207,150