Stockholders' Equity - Schedule of Stockholders Equity (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017
Class Of Stock [Line Items]
Balance			$ 1,781,806
Beginning Balance(in shares)			67,700,858
Issuance of common stock, net of shares withheld for taxes			$ 1,384
Issuance costs related to secondary offering of common stock			(474)
Stock-based compensation			7,835
Other comprehensive income (loss)			(1,971)
Net income (loss)	$ (66,655)	$ (9,356)	941	$ 11,074
Dividends on preferred shares, declared	(6,000)	0	(6,000)	0
Balance	$ 1,783,521	$ 1,349,168	$ 1,783,521	$ 1,349,168
Ending Balance(in shares)	68,043,284	60,255,320	68,043,284	60,255,320
Common Stock [Member]
Class Of Stock [Line Items]
Balance			$ 677
Beginning Balance(in shares)			67,700,858
Issuance of common stock, net of shares withheld for taxes			$ 3
Issuance of common stock, net of shares withheld for taxes (in shares)			342,426
Balance	$ 680		$ 680
Ending Balance(in shares)	68,043,284		68,043,284
Additional Paid-in Capital [Member]
Class Of Stock [Line Items]
Balance			$ 1,047,506
Issuance of common stock, net of shares withheld for taxes			1,381
Issuance costs related to secondary offering of common stock			(474)
Stock-based compensation			7,835
Balance	$ 1,056,248		1,056,248
Retained Earnings [Member]
Class Of Stock [Line Items]
Balance			748,186
Net income (loss)			941
Dividends on preferred shares, declared			(6,000)
Balance	743,127		743,127
Accumulated Other Comprehensive Loss [Member]
Class Of Stock [Line Items]
Balance			(14,563)
Other comprehensive income (loss)			(1,971)
Balance	$ (16,534)		$ (16,534)