Income Taxes (Reconciliation of U.S Statutory Rate to Effective Tax Rate) (Details)	3 Months Ended	9 Months Ended	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Income Taxes
Statutory tax rate	35.00%	21.00%	24.50%	35.00%	35.00%
State income taxes, net of federal tax benefit			(0.50%)	(0.30%)	0.40%
Taxes on international activities			(1.80%)	(7.60%)	(2.20%)
Research credit			(4.80%)	(3.20%)	(3.60%)
Net excess income tax benefit from stock-based compensation			(1.40%)	(1.40%)	(2.60%)
Domestic production activities deduction			(1.60%)	(1.50%)	(2.10%)
Adjustments of prior period tax items			(5.00%)	(0.90%)	(0.20%)
Effect of U.S. federal corporate rate reduction on net beginning U.S. deferred tax liability			(5.00%)
Transition Tax			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			1.60%	0.60%	(1.30%)
Retroactive extension of research credit					(3.20%)
Effective tax rate			17.90%	20.70%	20.20%