Consolidated Statements of Shareholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common stock, without par value [Member]	Retained earnings [Member]	Additional paid-in capital [Member]	Accumulated other comprehensive income (loss) [Member]
Balance, beginning of year at Sep. 30, 2009		$ 60,259	$ 201,710	$ 467,160	$ (83,086)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	90,258		90,258
Cash dividends — Common Stock			(74,008)
Dividends of net assets			0
Excess tax benefits on equity-based compensation				471
Net losses on derivative instruments	0				0
Reclassifications of net losses on derivative instruments	681				681
Benefit plans, principally actuarial losses					(15,699)
Reclassifications of benefit plans actuarial losses and prior service costs	3,406				3,406
Reclassification of pension plans actuarial losses and prior service costs to regulatory asset	83,302				83,302
Balance, end of year at Sep. 30, 2010	734,454	60,259	217,960	467,631	(11,396)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	105,196		105,196
Cash dividends — Common Stock			(99,490)
Dividends of net assets			(11,570)
Excess tax benefits on equity-based compensation				692
Net losses on derivative instruments	(10,874)				(10,874)
Reclassifications of net losses on derivative instruments	681				681
Benefit plans, principally actuarial losses					(674)
Reclassifications of benefit plans actuarial losses and prior service costs	430				430
Reclassification of pension plans actuarial losses and prior service costs to regulatory asset	0				0
Balance, end of year at Sep. 30, 2011	718,845	60,259	212,096	468,323	(21,833)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net Income	87,898		87,898
Cash dividends — Common Stock			(70,615)
Dividends of net assets			0
Excess tax benefits on equity-based compensation				369
Net losses on derivative instruments	(6,744)				(6,744)
Reclassifications of net losses on derivative instruments	1,111				1,111
Benefit plans, principally actuarial losses					(2,745)
Reclassifications of benefit plans actuarial losses and prior service costs	394				394
Reclassification of pension plans actuarial losses and prior service costs to regulatory asset	0				0
Balance, end of year at Sep. 30, 2012	$ 728,513	$ 60,259	$ 229,379	$ 468,692	$ (29,817)