SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) (USD $)	3 Months Ended								12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2012
SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED)
Operating revenues	$ 985,038,000	$ 952,087,000	$ 893,430,000	$ 889,152,000	$ 864,536,000	$ 840,197,000	$ 838,309,000	$ 844,572,000	$ 3,719,707,000	$ 3,387,614,000	$ 3,151,802,000
Operating income	263,502,000	271,912,000	255,342,000	264,031,000	243,234,000	239,960,000	232,920,000	240,547,000	1,054,787,000	956,661,000	909,599,000
Income from continuing operations	168,705,000	192,290,000	174,589,000	173,182,000	159,797,000	250,978,000	151,067,000	159,611,000	708,766,000	721,453,000	573,609,000
Net income	168,688,000	192,279,000	174,570,000	173,182,000	159,797,000	266,159,000	151,080,000	159,603,000	708,719,000	736,639,000	581,045,000
Basic earnings per common share:
Income from continuing operations (in dollars per share)	$ 1.55	$ 1.77	$ 1.61	$ 1.61	$ 1.49	$ 2.35	$ 1.41	$ 1.50	$ 6.54	$ 6.75	$ 5.35
Net income (in dollars per share)	$ 1.55	$ 1.77	$ 1.61	$ 1.61	$ 1.49	$ 2.49	$ 1.41	$ 1.50	$ 6.54	$ 6.89	$ 5.42
Diluted earnings per common share:
Income from continuing operations (in dollars per share)	$ 1.53	$ 1.75	$ 1.59	$ 1.59	$ 1.47	$ 2.32	$ 1.39	$ 1.48	$ 6.46	$ 6.65	$ 5.27
Net income (in dollars per share)	$ 1.53	$ 1.75	$ 1.59	$ 1.59	$ 1.47	$ 2.46	$ 1.39	$ 1.48	$ 6.46	$ 6.79	$ 5.34
Gain from the sale of assets, net	5,000,000	1,400,000	2,700,000	3,700,000	2,800,000	2,600,000	3,400,000	3,400,000
Gain from the sale of assets, per diluted share (in dollars per share)	$ 0.05	$ 0.01	$ 0.02	$ 0.03	$ 0.03	$ 0.02	$ 0.03	$ 0.03
Gain on sale of investment securities, net of tax		$ 14,900,000	$ 12,900,000			$ 92,400,000		$ 5,500,000
Gain from the sale of investment securities, per diluted share (in dollars per share)		$ 0.13	$ 0.12			$ 0.86		$ 0.05