Revenues - Reconciliation of ASC 606 to Prior Standards (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	Sep. 30, 2016
Consolidated Balance Sheets
Contract assets		$ 115,310	[1]				$ 53,034				$ 115,310	[1]	$ 53,034
Inventories		128,825					135,416				128,825		135,416
Total current assets		495,194					396,023				495,194		396,023
Total assets		1,466,720					1,265,122				1,466,720		1,265,122
Contract liabilities		81,177	[2]				49,035				81,177	[2]	49,035
Total current liabilities		251,635					200,530				251,635		200,530
Deferred tax liabilities		64,855					64,794				64,855		64,794
Total liabilities		640,498					505,712				640,498		505,712
Retained earnings		684,741					606,837				684,741		606,837
Total shareholders' equity		826,222					759,410				826,222		759,410	$ 671,918	$ 615,109
Total liabilities and shareholders' equity		1,466,720					1,265,122				1,466,720		1,265,122
Consolidated Statements of Operations
Net sales		236,658		$ 199,766	$ 193,949	$ 182,597	231,086	$ 192,223	$ 174,778	$ 173,495	812,970		771,582	685,740
Cost of sales											508,521		490,397	436,918
Total costs and expenses											710,754		683,559	605,587
Earnings before income tax											102,216		88,023	80,153
Income tax expense											21,177		(4,113)	26,450
Net earnings		$ 24,858		$ 20,067	$ 18,797	$ 17,317	$ 28,452	$ 19,019	$ 9,994	$ 34,671	$ 81,039		$ 92,136	$ 53,703
Basic:
Net earnings		$ 0.96		$ 0.77	$ 0.73	$ 0.67	$ 1.10	$ 0.73	$ 0.39	$ 1.34	$ 3.12		$ 3.56	$ 2.08
Diluted:
Net earnings		$ 0.95		$ 0.77	$ 0.72	$ 0.66	$ 1.09	$ 0.73	$ 0.38	$ 1.33	$ 3.10		$ 3.54	$ 2.07
Consolidated Statements of Comprehensive Income
Net earnings		$ 24,858		$ 20,067	$ 18,797	$ 17,317	$ 28,452	$ 19,019	$ 9,994	$ 34,671	$ 81,039		$ 92,136	$ 53,703
Comprehensive income											68,593		87,916	65,678
Consolidated Statements of Cash flows
Net earnings		24,858		$ 20,067	$ 18,797	$ 17,317	$ 28,452	$ 19,019	$ 9,994	$ 34,671	81,039		92,136	53,703
Adjustments to reconcile net earnings to net cash provided by operating activities:
Change in assets and liabilities											(9,944)		(10,315)	(17,889)
Net cash provided by operating activities											105,137		$ 93,259	$ 67,340
Effect of the adoption of ASC 606
Consolidated Balance Sheets
Contract assets	[1]	(39,055)									(39,055)
Inventories		34,065									34,065
Total current assets		(4,990)									(4,990)
Total assets		(4,990)									(4,990)
Contract liabilities	[2]	2,870									2,870
Total current liabilities		2,870									2,870
Deferred tax liabilities		(658)									(658)
Total liabilities		2,212									2,212
Retained earnings		(7,202)									(7,202)
Total shareholders' equity		(7,202)									(7,202)
Total liabilities and shareholders' equity		(4,990)									(4,990)
Consolidated Statements of Operations
Net sales											(5,598)
Cost of sales											(6,658)
Total costs and expenses											(6,658)
Earnings before income tax											1,060
Income tax expense											255
Net earnings											$ 805
Basic:
Net earnings											$ 0.03
Diluted:
Net earnings											$ 0.03
Consolidated Statements of Comprehensive Income
Net earnings											$ 805
Comprehensive income											805
Consolidated Statements of Cash flows
Net earnings											805
Adjustments to reconcile net earnings to net cash provided by operating activities:
Change in assets and liabilities											(805)
Net cash provided by operating activities											0
Under Prior Accounting
Consolidated Balance Sheets
Contract assets	[1]	76,255									76,255
Inventories		162,890									162,890
Total current assets		490,204									490,204
Total assets		1,461,730									1,461,730
Contract liabilities	[2]	84,047									84,047
Total current liabilities		254,505									254,505
Deferred tax liabilities		64,197									64,197
Total liabilities		642,710									642,710
Retained earnings		677,539									677,539
Total shareholders' equity		819,020									819,020
Total liabilities and shareholders' equity		$ 1,461,730									1,461,730
Consolidated Statements of Operations
Net sales											807,372
Cost of sales											501,863
Total costs and expenses											704,096
Earnings before income tax											103,276
Income tax expense											21,432
Net earnings											$ 81,844
Basic:
Net earnings											$ 3.15
Diluted:
Net earnings											$ 3.13
Consolidated Statements of Comprehensive Income
Net earnings											$ 81,844
Comprehensive income											69,398
Consolidated Statements of Cash flows
Net earnings											81,844
Adjustments to reconcile net earnings to net cash provided by operating activities:
Change in assets and liabilities											(10,749)
Net cash provided by operating activities											$ 105,137

[1] Previously “cost and estimated earnings on long-term contracts”

[2] Previously “advance payments on long-term contracts” and “current portion of deferred revenue”