Summary of Significant Accounting Policies - Net Income (Loss) Per Share (Details) - USD ($) shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Basic
Net income (loss)	$ 108,129	$ 9,259	$ (20,749)	$ 17,699	$ (44,508)	$ (20,720)	$ (167,141)	$ 47,465	$ 213,810	$ (159,928)	$ (150,996)
Diluted
Net income (loss) attributable to common stockholders - diluted	$ 108,129	$ 9,259	$ (20,749)	$ 17,699	$ (44,508)	$ (20,720)	$ (167,141)	$ 47,465	$ 213,810	$ (159,928)	$ (150,996)
Basic
Basic (in shares)	285,754	285,942	285,866	287,796	287,052	292,663	294,103	291,367	286,347	291,318	289,348