Shareholders' Equity - Components of Equity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	6 Months Ended
Mar. 31, 2020	Dec. 31, 2019	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2020	Mar. 31, 2019	Oct. 01, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)		119,338,925			119,338,925
Shareholders' equity, beginning balance	$ 6,127,775	$ 5,750,223	$ 5,348,195	$ 4,769,951	$ 5,750,223	$ 4,769,951
Net income	239,646	178,673	214,888	157,646	418,319	372,534
Other comprehensive income (loss)	890	1,052	(2,695)	(22,258)	$ 1,942	(24,953)
Cash dividends	(70,520)	(69,557)	(61,606)	(58,722)
Cumulative effect of accounting change							$ (8,210)
Public and other stock offerings	3,096	263,272	5,454	498,975
Stock-based compensation plans	$ 3,528	4,112	3,865	2,603
Common stock outstanding (in shares)	122,308,725				122,308,725
Shareholders' equity, ending balance	$ 6,304,415	$ 6,127,775	$ 5,508,101	$ 5,348,195	$ 6,304,415	$ 5,508,101
Cash dividends per share (USD per share)	$ 0.575	$ 0.575	$ 0.525	$ 0.525	$ 1.15	$ 1.05
Common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Common stock outstanding (in shares)	122,262,403	119,338,925	116,892,959	111,273,683	119,338,925	111,273,683
Shareholders' equity, beginning balance	$ 611	$ 597	$ 584	$ 556	$ 597	$ 556
Public and other stock offerings (in shares)	38,662	2,758,929	61,006	5,434,812
Public and other stock offerings	$ 1	$ 13	$ 1	$ 27
Stock-based compensation plans (in shares)	7,660	164,549	28,938	184,464
Stock-based compensation plans	$ 0	$ 1	$ 0	$ 1
Common stock outstanding (in shares)	122,308,725	122,262,403	116,982,903	116,892,959	122,308,725	116,982,903
Shareholders' equity, ending balance	$ 612	$ 611	$ 585	$ 584	$ 612	$ 585
Additional Paid-in Capital
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	3,979,564	3,712,194	3,476,476	2,974,926	3,712,194	2,974,926
Public and other stock offerings	3,095	263,259	5,453	498,948
Stock-based compensation plans	3,528	4,111	3,865	2,602
Shareholders' equity, ending balance	3,986,187	3,979,564	3,485,794	3,476,476	3,986,187	3,485,794
Accumulated Other Comprehensive Income (Loss)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	(113,531)	(114,583)	(114,115)	(83,647)	(114,583)	(83,647)
Other comprehensive income (loss)	890	1,052	(2,695)	(22,258)
Cumulative effect of accounting change							(8,210)
Shareholders' equity, ending balance	(112,641)	(113,531)	(116,810)	(114,115)	(112,641)	(116,810)
Retained Earnings
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Shareholders' equity, beginning balance	2,261,131	2,152,015	1,985,250	1,878,116	2,152,015	1,878,116
Net income	239,646	178,673	214,888	157,646
Cash dividends	(70,520)	(69,557)	(61,606)	(58,722)
Cumulative effect of accounting change							$ 8,210
Shareholders' equity, ending balance	$ 2,430,257	$ 2,261,131	$ 2,138,532	$ 1,985,250	$ 2,430,257	$ 2,138,532

Aggregate dividends paid during the period for each share of common stock outstanding.

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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Number of new stock issued during the period.

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Equity impact of the value of new stock issued during the period. Includes shares issued in an initial public offering or a secondary public offering.

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Total of all stockholders' equity (deficit) items, net of receivables from officers, directors, owners, and affiliates of the entity which are attributable to the parent. The amount of the economic entity's stockholders' equity attributable to the parent excludes the amount of stockholders' equity which is allocable to that ownership interest in subsidiary equity which is not attributable to the parent (noncontrolling interest, minority interest). This excludes temporary equity and is sometimes called permanent equity.

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