QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	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, 2011	Dec. 31, 2010	Dec. 31, 2009
QUARTERLY FINANCIAL DATA (UNAUDITED)
Total operating revenues	$ 1,036	$ 1,092	$ 1,000	$ 1,281	$ 1,146	$ 1,088	$ 939	$ 1,428	$ 4,409	$ 4,601	$ 4,237
Operating income	208	215	142	248	205	196	137	230	813	768	699
Income Available to common shareholders	$ 98	$ 105	$ 56	$ 128	$ 94	$ 101	$ 54	$ 127	$ 387	$ 376	$ 348
Basic earnings per share (in dollars per share)	$ 0.76	$ 0.81	$ 0.44	$ 1.00	$ 0.74	$ 0.80	$ 0.43	$ 1.02	$ 3.01	$ 2.99	$ 2.85
Diluted earnings per share (in dollars per share)	$ 0.75	$ 0.81	$ 0.43	$ 1.00	$ 0.74	$ 0.79	$ 0.43	$ 1.02	$ 2.97	$ 2.98	$ 2.85