CONSOLIDATING GUARANTOR AND NON-GUARANTOR FINANCIAL INFORMATION - Condensed Consolidating Balance Sheets (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018
CURRENT ASSETS
Cash and equivalents	$ 69,024		$ 72,377
Accounts receivable, net of allowances	335,033		264,450
Contract costs and recognized income not yet billed, net of progress payments	94,495		105,111
Inventories, net	462,119		442,121
Prepaid and other current assets	42,723		40,799
Assets of discontinued operations	321		321
Total Current Assets	1,003,715		925,179
PROPERTY, PLANT AND EQUIPMENT, net	335,820		337,326
OPERATING LEASE RIGHT-OF-USE ASSETS	156,258
GOODWILL	436,782		437,067
INTANGIBLE ASSETS, net	353,743		356,639
INTERCOMPANY RECEIVABLE	0		0
EQUITY INVESTMENTS IN SUBSIDIARIES	0		0
OTHER ASSETS	29,556		15,840
ASSETS OF DISCONTINUED OPERATIONS	2,873		2,888
Total Assets	2,318,747		2,074,939
CURRENT LIABILITIES
Notes payable and current portion of long-term debt	9,470		10,525
Accounts payable and accrued liabilities	347,153		375,241
Current portion of operating lease liabilities	28,047
Liabilities of discontinued operations	2,450		4,333
Total Current Liabilities	387,120		390,099
LONG-TERM DEBT, net	1,216,226		1,093,749
LONG-TERM OPERATING LEASE LIABILITIES	133,498
INTERCOMPANY PAYABLES	0		0
OTHER LIABILITIES	102,295		109,997
LIABILITIES OF DISCONTINUED OPERATIONS	3,154		3,331
Total Liabilities	1,842,293		1,597,176
SHAREHOLDERS’ EQUITY	476,454	$ 494,693	477,763	$ 474,284	$ 471,561	$ 474,391
Total Liabilities and Shareholders’ Equity	2,318,747		2,074,939
Reportable Legal Entities | Parent Company
CURRENT ASSETS
Cash and equivalents	9,252		1,649
Accounts receivable, net of allowances	0		0
Contract costs and recognized income not yet billed, net of progress payments	0		0
Inventories, net	0		0
Prepaid and other current assets	11,482		8,238
Assets of discontinued operations	0		0
Total Current Assets	20,734		9,887
PROPERTY, PLANT AND EQUIPMENT, net	1,295		1,184
OPERATING LEASE RIGHT-OF-USE ASSETS	10,060
GOODWILL	0		0
INTANGIBLE ASSETS, net	93		93
INTERCOMPANY RECEIVABLE	67,266		5,834
EQUITY INVESTMENTS IN SUBSIDIARIES	1,658,792		1,628,031
OTHER ASSETS	8,695		8,182
ASSETS OF DISCONTINUED OPERATIONS	0		0
Total Assets	1,766,935		1,653,211
CURRENT LIABILITIES
Notes payable and current portion of long-term debt	0		0
Accounts payable and accrued liabilities	28,873		41,796
Current portion of operating lease liabilities	1,830
Liabilities of discontinued operations	0		0
Total Current Liabilities	30,703		41,796
LONG-TERM DEBT, net	1,167,023		1,040,449
LONG-TERM OPERATING LEASE LIABILITIES	9,353
INTERCOMPANY PAYABLES	63,230		71,634
OTHER LIABILITIES	20,172		21,569
LIABILITIES OF DISCONTINUED OPERATIONS	0		0
Total Liabilities	1,290,481		1,175,448
SHAREHOLDERS’ EQUITY	476,454		477,763
Total Liabilities and Shareholders’ Equity	1,766,935		1,653,211
Reportable Legal Entities | Guarantor Companies
CURRENT ASSETS
Cash and equivalents	18,977		25,217
Accounts receivable, net of allowances	283,929		227,069
Contract costs and recognized income not yet billed, net of progress payments	91,998		104,109
Inventories, net	391,934		372,839
Prepaid and other current assets	23,468		25,754
Assets of discontinued operations	0		0
Total Current Assets	810,306		754,988
PROPERTY, PLANT AND EQUIPMENT, net	290,685		289,282
OPERATING LEASE RIGHT-OF-USE ASSETS	127,133
GOODWILL	375,734		375,734
INTANGIBLE ASSETS, net	220,796		224,275
INTERCOMPANY RECEIVABLE	928,899		864,884
EQUITY INVESTMENTS IN SUBSIDIARIES	745,746		581,438
OTHER ASSETS	37,669		24,635
ASSETS OF DISCONTINUED OPERATIONS	0		0
Total Assets	3,536,968		3,115,236
CURRENT LIABILITIES
Notes payable and current portion of long-term debt	3,474		3,075
Accounts payable and accrued liabilities	412,118		266,411
Current portion of operating lease liabilities	22,068
Liabilities of discontinued operations	0		0
Total Current Liabilities	437,660		269,486
LONG-TERM DEBT, net	10,642		3,119
LONG-TERM OPERATING LEASE LIABILITIES	108,872
INTERCOMPANY PAYABLES	563,964		457,265
OTHER LIABILITIES	77,292		81,582
LIABILITIES OF DISCONTINUED OPERATIONS	0		0
Total Liabilities	1,198,430		811,452
SHAREHOLDERS’ EQUITY	2,338,538		2,303,784
Total Liabilities and Shareholders’ Equity	3,536,968		3,115,236
Reportable Legal Entities | Non-Guarantor Companies
CURRENT ASSETS
Cash and equivalents	40,795		45,511
Accounts receivable, net of allowances	206,456		38,580
Contract costs and recognized income not yet billed, net of progress payments	2,497		1,002
Inventories, net	70,756		69,540
Prepaid and other current assets	5,149		6,951
Assets of discontinued operations	321		321
Total Current Assets	325,974		161,905
PROPERTY, PLANT AND EQUIPMENT, net	43,840		46,860
OPERATING LEASE RIGHT-OF-USE ASSETS	19,065
GOODWILL	61,048		61,333
INTANGIBLE ASSETS, net	132,854		132,271
INTERCOMPANY RECEIVABLE	148,568		75,684
EQUITY INVESTMENTS IN SUBSIDIARIES	3,109,559		3,233,038
OTHER ASSETS	(2,446)		(2,352)
ASSETS OF DISCONTINUED OPERATIONS	2,873		2,888
Total Assets	3,841,335		3,711,627
CURRENT LIABILITIES
Notes payable and current portion of long-term debt	5,996		7,450
Accounts payable and accrued liabilities	58,903		68,390
Current portion of operating lease liabilities	4,149
Liabilities of discontinued operations	2,450		4,333
Total Current Liabilities	71,498		80,173
LONG-TERM DEBT, net	38,561		50,181
LONG-TERM OPERATING LEASE LIABILITIES	15,273
INTERCOMPANY PAYABLES	536,815		444,557
OTHER LIABILITIES	12,650		15,017
LIABILITIES OF DISCONTINUED OPERATIONS	3,154		3,331
Total Liabilities	677,951		593,259
SHAREHOLDERS’ EQUITY	3,163,384		3,118,368
Total Liabilities and Shareholders’ Equity	3,841,335		3,711,627
Elimination
CURRENT ASSETS
Cash and equivalents	0		0
Accounts receivable, net of allowances	(155,352)		(1,199)
Contract costs and recognized income not yet billed, net of progress payments	0		0
Inventories, net	(571)		(258)
Prepaid and other current assets	2,624		(144)
Assets of discontinued operations	0		0
Total Current Assets	(153,299)		(1,601)
PROPERTY, PLANT AND EQUIPMENT, net	0		0
OPERATING LEASE RIGHT-OF-USE ASSETS	0
GOODWILL	0		0
INTANGIBLE ASSETS, net	0		0
INTERCOMPANY RECEIVABLE	(1,144,733)		(946,402)
EQUITY INVESTMENTS IN SUBSIDIARIES	(5,514,097)		(5,442,507)
OTHER ASSETS	(14,362)		(14,625)
ASSETS OF DISCONTINUED OPERATIONS	0		0
Total Assets	(6,826,491)		(6,405,135)
CURRENT LIABILITIES
Notes payable and current portion of long-term debt	0		0
Accounts payable and accrued liabilities	(152,741)		(1,356)
Current portion of operating lease liabilities	0
Liabilities of discontinued operations	0		0
Total Current Liabilities	(152,741)		(1,356)
LONG-TERM DEBT, net	0		0
LONG-TERM OPERATING LEASE LIABILITIES	0
INTERCOMPANY PAYABLES	(1,164,009)		(973,456)
OTHER LIABILITIES	(7,819)		(8,171)
LIABILITIES OF DISCONTINUED OPERATIONS	0		0
Total Liabilities	(1,324,569)		(982,983)
SHAREHOLDERS’ EQUITY	(5,501,922)		(5,422,152)
Total Liabilities and Shareholders’ Equity	$ (6,826,491)		$ (6,405,135)