Stockholders' Equity (Details) - USD ($) $ in Thousands	3 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Balance	$ 1,323,827
Issuance of common stock, net of shares withheld for taxes	3,164
Stock-based compensation	3,816
Other comprehensive income	(1,393)
Net income	20,430	$ 7,118
Balance	1,349,844	$ 1,211,153
Common Stock [Member]
Balance	$ 598
Balance (in shares)	59,890,885
Issuance of common stock, net of shares withheld for taxes	$ 2
Issuance of common stock, net of shares withheld for taxes (Shares)	139,631
Stock-based compensation	$ 0
Other comprehensive income	0
Net income	0
Balance	$ 600
Balance (in shares)	60,030,516
Additional Paid-in Capital [Member]
Balance	$ 694,564
Issuance of common stock, net of shares withheld for taxes	3,162
Stock-based compensation	3,816
Other comprehensive income	0
Net income	0
Balance	701,542
Retained Earnings [Member]
Balance	647,322
Issuance of common stock, net of shares withheld for taxes	0
Stock-based compensation	0
Other comprehensive income	0
Net income	20,430
Balance	667,752
Accumulated Other Comprehensive Income (Loss)
Balance	(18,657)
Issuance of common stock, net of shares withheld for taxes	0
Stock-based compensation	0
Other comprehensive income	(1,393)
Net income	0
Balance	$ (20,050)