Statement of Owner's Equity (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
EquityAbstract
Owner's capital - balance at beginning of period	$ 1,051,962	$ 1,046,862	$ 1,027,862
Capital contributions from parent	8,630	5,100	19,000
Owner's capital - balance at end of period	1,060,592	1,051,962	1,046,862
Loans (to) from affiliate - balance at beginning of period	0	0	(105,431)
Loans (to) from affiliate	0	0	105,431
Loans (to) from affiliate - balance at end of period	0	0	0
Retained earnings - balance at beginning of period	268,209	242,396	284,319
Net income	136,723	152,813	149,527
Cash distributions	(137,500)	(127,000)	(191,450)
Retained earnings - balance at end of period	267,432	268,209	242,396
Accumulated other comprehensive income (loss) - balance at beginning of period	276	338	400
Cash flow hedges: Reclassification of gain into earnings	(62)	(62)	(62)
Accumulated other comprehensive income (loss) - balance at end of period	214	276	338
Total owner's equity	$ 1,328,238	$ 1,320,447	$ 1,289,596