Nature of Business and Summary of Significant Accounting Policies - New Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2017		Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Oct. 01, 2019
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																	$ 1,160,083	$ 1,000,146	$ 934,983
Cost of revenues	$ 87,996	[1]	$ 87,215	[1]	$ 85,568	[1]	$ 76,066	[1]	$ 79,962	[1]	$ 79,011	[1]	$ 79,493	[1]	$ 74,432	[1]	336,845	312,898	287,607
Selling, general and administrative																	414,086	376,912	337,167
Provision for income taxes																	23,948	30,450	22,869
Net income	$ 54,584		$ 64,152		$ 33,381		$ 40,007		$ 32,713		$ 29,721		$ 31,169		$ 32,879		192,124	126,482	133,414
Comprehensive Income (Loss), Net of Tax, Attributable to Parent																	$ 178,460	$ 116,556	$ 143,931
Basic earnings per share (in dollars per share)	$ 1.89	[2]	$ 2.21	[2]	$ 1.15	[2]	$ 1.38	[2]	$ 1.13	[2]	$ 1.00	[2]	$ 1.04	[2]	$ 1.09	[2]	$ 6.63	$ 4.26	$ 4.32
Diluted earnings per share (in dollars per share)	$ 1.80	[2]	$ 2.12	[2]	$ 1.10	[2]	$ 1.32	[2]	$ 1.07	[2]	$ 0.95	[2]	$ 1.00	[2]	$ 1.04	[2]	$ 6.34	$ 4.06	$ 4.14
Deferred income taxes																	$ 7,701	$ 10,584	$ (6,248)
Increase (Decrease) in Operating Capital																		(8,415)	(612)
Accounting Standards Update 2016-02 | Forecast [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Lease, Liability																				$ 99,000
Operating Lease, Right-of-Use Asset																				$ 90,000
As Previously Reported
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		1,032,475	932,169
Cost of revenues																		310,699	287,123
Selling, general and administrative																		380,362	339,796
Provision for income taxes																		45,595	23,068
Net income																		142,415	128,256
Comprehensive Income (Loss), Net of Tax, Attributable to Parent																		$ 132,502	$ 138,773
Basic earnings per share (in dollars per share)																		$ 4.79	$ 4.16
Diluted earnings per share (in dollars per share)																		$ 4.57	$ 3.98
Deferred income taxes																		$ 25,729	$ (6,049)
Increase (Decrease) in Operating Capital																		(39,493)	4,347
Adjustment | Accounting Standards Update 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenue from Contract with Customer, Excluding Assessed Tax																		(32,329)	2,814
Cost of revenues																		2,199	484
Selling, general and administrative																		(3,450)	(2,629)
Provision for income taxes																		(15,145)	(199)
Net income																		(15,933)	5,158
Comprehensive Income (Loss), Net of Tax, Attributable to Parent																		$ (15,946)	$ 5,158
Basic earnings per share (in dollars per share)																		$ (0.53)	$ 0.16
Diluted earnings per share (in dollars per share)																		$ (0.51)	$ 0.16
Deferred income taxes																		$ (15,145)	$ (199)
Increase (Decrease) in Operating Capital																		$ 31,078	$ (4,959)

[1] Cost of revenues excludes amortization expense of $0.5 million, $0.5 million, $0.5 million, $0.5 million, $0.5 million, $0.6 million, $0.6 million and $0.7 million for the quarters ended September 30, 2019, June 30, 2019, March 31, 2019, December 31, 2018, September 30, 2018, June 30, 2018, March 31, 2018 and December 31, 2017, respectively.

[2] 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.