Income Taxes (Reconciliation of U.S Statutory Rate to Effective Tax Rate) (Details)			12 Months Ended			21 Months Ended
	Jan. 01, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2019
Income Taxes
Statutory tax rate	21.00%	35.00%	21.00%	24.50%	35.00%	21.00%
State income taxes, net of federal tax benefit			(0.10%)	(0.50%)	(0.30%)
Taxes on international activities			0.20%	(1.80%)	(7.60%)
Research credit			(3.30%)	(4.80%)	(3.20%)
Net excess income tax benefit from stock-based compensation			(3.50%)	(1.40%)	(1.40%)
Domestic production activities deduction				(1.60%)	(1.50%)
Adjustments of prior period tax items			0.90%	(5.00%)	(0.90%)
Effect of U.S. federal corporate rate reduction on net beginning U.S. deferred tax liability				(5.00%)
Transition Tax			3.30%	6.20%
Increased deferred tax liability associated with anticipated repatriation taxes				3.70%
Effect of US federal corporate rate reduction on net current year US deferred tax activity				2.00%
Other items, net			0.50%	1.60%	0.60%
Effective tax rate			19.00%	17.90%	20.70%