Supplemental Guarantor Information - Condensed Consolidated Statements of Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Net income	$ 67,596	$ 20,430
Other comprehensive income (loss):
Foreign currency translation adjustment	57	(1,652)
Total other comprehensive income (loss)	57	(1,652)
Comprehensive income	67,653	18,778
Parent Company [Member]
Net income	67,596	20,430
Other comprehensive income (loss):
Foreign currency translation adjustment	57	(1,652)
Total other comprehensive income (loss)	57	(1,652)
Comprehensive income	67,653	18,778
Guarantor Subsidiaries [Member]
Net income	90,490	24,279
Other comprehensive income (loss):
Foreign currency translation adjustment	0	0
Total other comprehensive income (loss)	0	0
Comprehensive income	90,490	24,279
Non-Guarantor Subsidiaries [Member]
Net income	(11,789)	(249)
Other comprehensive income (loss):
Foreign currency translation adjustment	57	(1,652)
Total other comprehensive income (loss)	57	(1,652)
Comprehensive income	(11,732)	(1,901)
Eliminations and Other [Member]
Net income	(78,701)	(24,030)
Other comprehensive income (loss):
Foreign currency translation adjustment	(57)	1,652
Total other comprehensive income (loss)	(57)	1,652
Comprehensive income	$ (78,758)	$ (22,378)