Income Taxes Income Tax Rate Reconciliation (Details)	3 Months Ended	6 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
Adjustments to reconcile to the effective tax rate:
State income taxes, net of federal benefit	4.50%	4.50%	4.50%	1.80%
Valuation allowance adjustment related to stock compensation	(0.60%)	0.00%	(0.60%)	0.00%
Tax benefits from stock compensation	(0.50%)	(0.30%)	(0.80%)	(2.50%)
Tax credits	(1.50%)	(1.30%)	(1.40%)	(2.60%)
Effective Income Tax Rate Reconciliation, GILTI, percent	(0.20%)	0.50%	0.00%	(1.10%)
Other	0.00%	1.40%	0.40%	(1.90%)
Walter Energy accrual	0.00%	0.00%	0.00%	(12.90%)
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Percent	(0.50%)	0.00%	(0.60%)	0.00%
Effective Income Tax Rate Reconciliation, Transition Tax	0.00%	0.00%	0.00%	4.70%

Effective Income Tax Rate Reconciliation, GILTI, percent

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Effective Income Tax Rate Reconciliation, Transition Tax

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deductions. Including, but not limited to, extraterritorial income exclusion deduction, qualified production activity deduction, dividend deduction, deduction for dividend paid to employee stock ownership plan, Medicare prescription drug benefit subsidy deduction, and other deductions.

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Percentage of the difference, between reported income tax expense (benefit) and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations, that is attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority interest income (expense), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, change in enacted tax rate, prior year income taxes, change in deferred tax asset valuation allowance, and other adjustments.

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