Income Taxes (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Unrecognized Tax Benefits	$ 3.8		$ 3.8		$ 3.3
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%	21.00%	21.00%	21.00%
State income taxes, net of federal benefit	4.50%	4.50%	4.50%	1.80%
Tax benefits from stock compensation	(0.50%)	(0.30%)	(0.80%)	(2.50%)
Tax credits	(1.50%)	(1.30%)	(1.40%)	(2.60%)
Other	0.00%	1.40%	0.40%	(1.90%)
Walter Energy accrual	0.00%	0.00%	0.00%	(12.90%)