Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended							6 Months Ended	9 Months Ended		12 Months Ended				18 Months Ended	21 Months Ended
	Dec. 31, 2015	Oct. 04, 2015	Jul. 04, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 28, 2014	Jun. 29, 2014	Oct. 04, 2015	Dec. 31, 2015		Mar. 31, 2015		Mar. 31, 2014		Sep. 28, 2014	Dec. 31, 2014
Current:
Federal									$ 27,068		$ 21,321		$ 43,358
State									2,258		3,508		1,562
Deferred:
Federal									55,301		17,314		(110,792)
State									2,286		(4,844)		(8,547)
Income taxes	$ 21,127	$ 37,980	$ 27,806	$ (4,223)	$ 12,748	$ 13,763	$ 15,011	$ 65,785	$ 86,913	[1]	$ 37,299	[1]	$ (74,419)	[1]	$ 28,774	$ 41,521
Items responsible for the differences between the federal statutory rate and ATK's effective rate
Statutory federal income tax rate									35.00%		35.00%		35.00%
State income taxes, net of federal impact									2.10%		(0.10%)		7.20%
Domestic manufacturing deduction									(1.50%)		(5.60%)		3.90%
Goodwill impairment									0.00%		11.20%		0.00%
Research and development tax credit									(5.20%)		(3.20%)		2.70%
Change in prior year contingent tax liabilities									1.00%		(3.70%)		7.10%
Nondeductible transaction costs									0.00%		7.20%		0.00%
Other									(1.10%)		(3.30%)		(1.90%)
Change in valuation allowance									1.60%		(2.70%)		(0.30%)
Income tax provision									31.90%		34.80%		53.70%
Components of Deferred Tax Assets [Abstract]
Retirement benefits	306,008			344,674					$ 306,008		$ 344,674
Federal carryforwards	35,957			10,269					35,957		10,269
Other	42,733			49,082					42,733		49,082
Other reserves	25,676			28,697					25,676		28,697
Accruals for employee benefits	41,719			46,691					41,719		46,691
Inventory	14,206			10,893					14,206		10,893
Contract method of revenue recognition	108,513			134,329					108,513		134,329
Total deferred income tax assets before valuation allowance	574,812			624,635					574,812		624,635
Valuation allowance	(13,190)			(8,836)					(13,190)		(8,836)
Total deferred income tax assets	561,622			615,799					561,622		615,799
Components of Deferred Tax Liabilities [Abstract]
Intangible assets	(98,300)			(114,598)					(98,300)		(114,598)
Property, plant and equipment	(129,188)			(123,154)					(129,188)		(123,154)
Debt-related	(15,596)			(21,290)					(15,596)		(21,290)
Total deferred income tax liabilities	(243,084)			(259,042)					(243,084)		(259,042)
Net deferred income tax assets	$ 318,538			$ 356,757					$ 318,538		$ 356,757

[1] See Note 2 - Restatement