Quarterly Data (unaudited) (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Quarterly Financial Data [Abstract]
Revenues	$ 126,081	$ 159,934	$ 483,261	$ 323,105	$ 119,543	$ 146,692	$ 359,940	$ 261,413	$ 1,092,381	$ 887,588	$ 768,484
Operating income	2,186	3,914	135,127	96,295	1,992	27,671	116,408	82,236	237,522	228,307	200,901
Net income (loss)	$ (5,543)	$ (3,023)	$ 89,184	$ 68,303	$ (4,046)	$ 10,697	$ 65,125	$ 44,265	$ 148,921	$ 116,041	$ 97,373