Earnings per Share - Reconciliation of Numerators and Denominators of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Numerator for basic and diluted earnings per share
Net income	$ 59,126		$ 64,076		$ 58,288		$ 54,921		$ 54,584		$ 64,152		$ 33,381		$ 40,007		$ 236,411	$ 192,124	$ 126,482
Denominator — share:
Basic weighted-average shares (in shares)	29,045		29,005		29,194		29,025		28,918		28,967		29,074		28,961		29,067	28,980	29,711
Effect of dilutive securities (in shares)																	865	1,314	1,469
Diluted weighted-average shares (in shares)	29,833		29,744		29,985		30,169		30,290		30,292		30,259		30,336		29,932	30,294	31,180
Earnings per share:
Basic earnings per share (in dollars per share)	$ 2.04	[1]	$ 2.21	[1]	$ 2.00	[1]	$ 1.89	[1]	$ 1.89	[1]	$ 2.21	[1]	$ 1.15	[1]	$ 1.38	[1]	$ 8.13	$ 6.63	$ 4.26
Diluted earnings per share (in dollars per share)	$ 1.98	[1]	$ 2.15	[1]	$ 1.94	[1]	$ 1.82	[1]	$ 1.80	[1]	$ 2.12	[1]	$ 1.10	[1]	$ 1.32	[1]	$ 7.90	$ 6.34	$ 4.06

[1] Earnings per share is computed independently for each of the quarters presented. Therefore, the sum of the quarterly per share amounts may not equal the totals for the respective years.