Condensed Consolidating Financial Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 27, 2018	Sep. 28, 2017	Sep. 27, 2018	Sep. 28, 2017	Dec. 31, 2017	Dec. 31, 2016
Condensed Consolidating Statements of Operations [Abstract]
Revenue	$ 1,813.7	$ 1,748.2	$ 5,386.7	$ 5,268.4
Operating costs and expenses
Cost of sales	1,543.1	1,486.6	4,601.3	4,762.8
Selling, general and administrative	37.3	49.8	154.5	149.9
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds	0.0	0.0	0.0	19.9
Research and development	10.8	9.5	31.3	21.2
Total operating costs and expenses	1,591.2	1,545.9	4,787.1	4,953.8
Operating income	222.5	202.3	599.6	314.6
Interest expense and financing fee amortization	(24.2)	(10.4)	(60.3)	(30.1)
Other expense, net	7.4	11.0	(0.8)	32.2
Income (loss) before income taxes and equity in net loss of affiliates and subsidiaries	205.7	202.9	538.5	316.7
Income tax provision	(36.9)	(55.9)	(99.7)	(84.9)
Income (loss) Before Equity In Net Income (Loss) Of Affiliates and Subsidiaries	168.8	147.0	438.8	231.8
Equity in net income of affiliate	0.0	0.2	0.6	0.3
Equity in net income of subsidiaries	0.0		0.0
Net income	168.8	147.2	439.4	232.1
Total other comprehensive income (loss)	(0.6)	13.8	(16.5)	33.6
Comprehensive Income, Net of Tax, Attributable to Parent	168.2	161.0	422.9	265.7
Assets
Cash and cash equivalents	703.9	749.8	703.9	749.8	$ 445.5	$ 717.6
Restricted Cash and Cash Equivalents	0.3	3.2	0.3	3.2	2.2	0.0
Accounts receivable, net	685.1		685.1		722.2
Unbilled Receivables, Current	513.2		513.2		0.0
Inventory	931.0		931.0		1,449.9
Other current assets	35.4		35.4		53.5
Total current assets	2,848.4		2,848.4		2,651.1
Property, plant and equipment, net	2,123.0		2,123.0		2,105.3
Unbilled Receivable, Non Current	53.5		53.5		0.0
Pension assets	372.3		372.3		347.1
Other assets	284.9		284.9		164.3
Total assets	5,682.1		5,682.1		5,267.8
Liabilities
Accounts payable	882.4		882.4		693.1
Accrued expenses	354.0		354.0		269.3
Profit sharing	42.0		42.0		109.5
Current portion of long-term debt	25.0		25.0		31.1
Advance payments, short-term	112.7		112.7		100.0
Billings in Excess of Cost, Current	153.5		153.5		0.0
Provision for Loss on Contracts	147.3		147.3		0.0
Deferred revenue and other deferred credits, short-term	7.1		7.1		64.6
Deferred grant income liability - current	19.4		19.4		21.6
Other current liabilities	68.8		68.8		331.8
Total current liabilities	1,812.2		1,812.2		1,621.0
Long-term debt	1,869.7		1,869.7		1,119.9
Advance payments, long-term	146.4		146.4		231.7
Pension/OPEB obligation	38.4		38.4		40.8
Billings in Excess of Cost, Noncurrent	353.8		353.8		0.0
Provision for Loss on Contacts, Non Current	72.6		72.6		0.0
Deferred grant income liability - non-current	30.7		30.7		39.3
Deferred revenue and other deferred credits	26.8		26.8		161.0
Other liabilities	215.1		215.1		252.6
Total equity	1,116.4		1,116.4		1,801.5
Total liabilities and equity	5,682.1		5,682.1		5,267.8
Operating activities
Net cash (used in) operating activities			567.4	624.6
Investing activities
Purchase of property, plant and equipment			(170.9)	(138.7)
Proceeds from Sales of Assets, Investing Activities				0.4
Payments for (Proceeds from) Other Investing Activities			2.8	0.0
Net cash used in investing activities			(168.1)	(138.3)
Proceeds from Issuance of Senior Long-term Debt			1,300.0	0.0
Financing activities
Principal payments of debt			(4.9)	(2.3)
Repayments of Senior Debt			(256.3)	(12.5)
Early Repayment of Senior Debt			(300.0)	0.0
Repayments of Debt			(256.3)	(12.5)
Payments Related to Tax Withholding for Share-based Compensation			(15.5)	(13.8)
Debt issuance and financing costs			(23.2)	(0.9)
Increase (Decrease) in Other Operating Assets and Liabilities, Net			(28.3)	208.5
Proceeds from Other Debt			0.0	7.6
Proceeds from (Payments for) Other Financing Activities			0.0	0.0
Payments for Repurchase of Common Stock			(805.8)	(402.1)
Proceeds from subsidiary (payments to Parent) to pay dividends			0.0	0.0
Payments of Dividends			(35.4)	(35.7)
Net cash used in financing activities			(141.1)	(459.7)
Effect of exchange rate changes on cash and cash equivalents			0.2	5.6
Net increase in cash, cash equivalents, and restricted cash for the period			258.4	32.2
Cash and cash equivalents, beginning of period			423.3	697.7
Cash, cash equivalents, and restricted cash, end of period	683.4	726.6	683.4	726.6
Consolidation, Eliminations [Member]
Condensed Consolidating Statements of Operations [Abstract]
Revenue	(128.0)	(136.6)	(462.2)	(458.1)
Operating costs and expenses
Cost of sales	(128.0)	(136.6)	(462.2)	(458.1)
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds		0.0		0.0
Total operating costs and expenses	(128.0)	(136.6)	(462.2)	(458.1)
Interest expense and financing fee amortization	1.2	1.4	3.8	4.5
Other expense, net	(1.2)	(1.4)	(3.8)	(4.5)
Equity in net income of affiliate		(0.2)	(0.6)	(0.3)
Equity in net income of subsidiaries	(190.1)	(167.8)	(510.1)	(314.7)
Net income	(190.1)	(168.0)	(510.7)	(315.0)
Total other comprehensive income (loss)	0.5	(28.1)	31.2	(68.6)
Comprehensive Income, Net of Tax, Attributable to Parent	(189.6)	(196.1)	(479.5)	(383.6)
Assets
Restricted Cash and Cash Equivalents	0.0		0.0		0.0
Accounts receivable, net	(367.7)		(367.7)		(361.3)
Unbilled Receivables, Current	0.0		0.0
Total current assets	(367.7)		(367.7)		(361.3)
Unbilled Receivable, Non Current	0.0		0.0
Investment in subsidiary	(1,802.9)		(1,802.9)		(2,505.9)
Other assets	(218.4)		(218.4)		(258.4)
Total assets	(2,389.0)		(2,389.0)		(3,125.6)
Liabilities
Accounts payable	(367.7)		(367.7)		(361.3)
Billings in Excess of Cost, Current	0.0		0.0
Provision for Loss on Contracts	0.0		0.0
Total current liabilities	(367.7)		(367.7)		(361.3)
Long-term debt	(117.8)		(117.8)		(157.8)
Billings in Excess of Cost, Noncurrent	0.0		0.0
Provision for Loss on Contacts, Non Current	0.0		0.0
Other liabilities	(100.6)		(100.6)		(100.6)
Total equity	(1,802.9)		(1,802.9)		(2,505.9)
Total liabilities and equity	(2,389.0)		(2,389.0)		(3,125.6)
Investing activities
Proceeds from Sales of Assets, Investing Activities				0.0
Payments for (Proceeds from) Other Investing Activities			0.0	0.0
Proceeds from Issuance of Senior Long-term Debt			0.0
Financing activities
Repayments of Senior Debt			0.0	0.0
Early Repayment of Senior Debt			0.0
Payments Related to Tax Withholding for Share-based Compensation			0.0	0.0
Debt issuance and financing costs			0.0
Proceeds from Other Debt				0.0
Proceeds from (Payments for) Other Financing Activities			0.0	0.0
Payments for Repurchase of Common Stock				0.0
Proceeds from subsidiary (payments to Parent) to pay dividends			0.0	0.0
Payments of Dividends			0.0	0.0
Parent Company [Member]
Operating costs and expenses
Selling, general and administrative	2.3	3.9	8.0	8.8
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds		0.0		0.0
Total operating costs and expenses	2.3	3.9	8.0	8.8
Operating income	(2.3)	(3.9)	(8.0)	(8.8)
Income (loss) before income taxes and equity in net loss of affiliates and subsidiaries	(2.3)	(3.9)	(8.0)	(8.8)
Income tax provision	0.4	0.8	1.5	2.7
Income (loss) Before Equity In Net Income (Loss) Of Affiliates and Subsidiaries	(1.9)	(3.1)	(6.5)	(6.1)
Equity in net income of affiliate		0.2	0.6	0.3
Equity in net income of subsidiaries	170.7	150.1	445.3	237.9
Net income	168.8	147.2	439.4	232.1
Total other comprehensive income (loss)	(0.6)	13.8	(16.5)	33.6
Comprehensive Income, Net of Tax, Attributable to Parent	168.2	161.0	422.9	265.7
Assets
Restricted Cash and Cash Equivalents	0.0		0.0		0.0
Unbilled Receivables, Current	0.0		0.0
Unbilled Receivable, Non Current	0.0		0.0
Investment in subsidiary	1,116.4		1,116.4		1,801.5
Total assets	1,116.4		1,116.4		1,801.5
Liabilities
Billings in Excess of Cost, Current	0.0		0.0
Provision for Loss on Contracts	0.0		0.0
Billings in Excess of Cost, Noncurrent	0.0		0.0
Provision for Loss on Contacts, Non Current	0.0		0.0
Total equity	1,116.4		1,116.4		1,801.5
Total liabilities and equity	1,116.4		1,116.4		1,801.5
Investing activities
Proceeds from Sales of Assets, Investing Activities				0.0
Payments for (Proceeds from) Other Investing Activities			0.0	0.0
Proceeds from Issuance of Senior Long-term Debt			0.0
Financing activities
Repayments of Senior Debt			0.0	0.0
Early Repayment of Senior Debt			0.0
Payments Related to Tax Withholding for Share-based Compensation			0.0	0.0
Debt issuance and financing costs			0.0
Proceeds from Other Debt				0.0
Proceeds from (Payments for) Other Financing Activities			805.8	402.1
Payments for Repurchase of Common Stock			(805.8)	(402.1)
Proceeds from subsidiary (payments to Parent) to pay dividends			35.4	35.7
Payments of Dividends			(35.4)	(35.7)
Non-Guarantor Subsidiaries [Member]
Condensed Consolidating Statements of Operations [Abstract]
Revenue	305.6	311.8	1,017.6	1,025.4
Operating costs and expenses
Cost of sales	275.2	284.0	915.7	913.5
Selling, general and administrative	3.6	3.6	13.4	10.7
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds		0.0		0.0
Research and development	1.6	0.7	3.5	1.8
Total operating costs and expenses	280.4	288.3	932.6	926.0
Operating income	25.2	23.5	85.0	99.4
Interest expense and financing fee amortization	(1.2)	(1.3)	(4.0)	(4.5)
Other expense, net	(1.0)	(0.7)	(3.6)	(0.7)
Income (loss) before income taxes and equity in net loss of affiliates and subsidiaries	23.0	21.5	77.4	94.2
Income tax provision	(3.6)	(3.7)	(12.6)	(17.4)
Income (loss) Before Equity In Net Income (Loss) Of Affiliates and Subsidiaries	19.4	17.8	64.8	76.8
Equity in net income of affiliate		0.2	0.6	0.3
Net income	19.4	18.0	65.4	77.1
Total other comprehensive income (loss)	0.1	14.3	(14.7)	35.0
Comprehensive Income, Net of Tax, Attributable to Parent	19.5	32.3	50.7	112.1
Assets
Cash and cash equivalents	61.9	38.8	61.9	38.8	58.2	17.6
Restricted Cash and Cash Equivalents	0.0		0.0		0.0
Accounts receivable, net	322.7		322.7		330.9
Unbilled Receivables, Current	45.5		45.5
Inventory	320.9		320.9		439.9
Other current assets	3.0		3.0		3.2
Total current assets	754.0		754.0		832.2
Property, plant and equipment, net	501.9		501.9		519.5
Unbilled Receivable, Non Current	0.0		0.0
Pension assets	21.0		21.0		19.9
Other assets	130.5		130.5		124.5
Total assets	1,407.4		1,407.4		1,496.1
Liabilities
Accounts payable	412.5		412.5		425.4
Accrued expenses	36.4		36.4		29.8
Profit sharing	3.6		3.6		6.1
Current portion of long-term debt	0.9		0.9		0.9
Billings in Excess of Cost, Current	0.0		0.0
Provision for Loss on Contracts	0.0		0.0
Deferred revenue and other deferred credits, short-term	0.5		0.5		1.0
Deferred grant income liability - current	19.4		19.4		21.6
Other current liabilities					7.5
Total current liabilities	473.3		473.3		492.3
Long-term debt	125.5		125.5		167.1
Advance payments, long-term	2.2		2.2
Billings in Excess of Cost, Noncurrent	0.0		0.0
Provision for Loss on Contacts, Non Current	0.0		0.0
Deferred grant income liability - non-current	24.9		24.9		39.3
Deferred revenue and other deferred credits	2.5		2.5		2.8
Other liabilities	12.5		12.5		10.1
Total equity	766.5		766.5		784.5
Total liabilities and equity	1,407.4		1,407.4		1,496.1
Operating activities
Net cash (used in) operating activities			81.3	98.9
Investing activities
Purchase of property, plant and equipment			(28.0)	(16.6)
Proceeds from Sales of Assets, Investing Activities				0.0
Payments for (Proceeds from) Other Investing Activities			0.5	0.0
Net cash used in investing activities			(27.5)	(16.6)
Proceeds from Issuance of Senior Long-term Debt			0.0
Financing activities
Principal payments of debt			(0.6)	(1.6)
Repayments of Senior Debt				0.0
Early Repayment of Senior Debt			0.0
Collection on (repayment of) intercompany debt			(49.7)	(65.1)
Payments Related to Tax Withholding for Share-based Compensation			0.0	0.0
Debt issuance and financing costs			0.0
Proceeds from Other Debt				0.0
Proceeds from (Payments for) Other Financing Activities			0.0	0.0
Payments for Repurchase of Common Stock				0.0
Proceeds from subsidiary (payments to Parent) to pay dividends			0.0	0.0
Payments of Dividends			0.0	0.0
Net cash used in financing activities			(50.3)	(66.7)
Effect of exchange rate changes on cash and cash equivalents			0.2	5.6
Net increase in cash, cash equivalents, and restricted cash for the period			3.7	21.2
Cash and cash equivalents, beginning of period			58.2
Cash, cash equivalents, and restricted cash, end of period	61.9		61.9
Subsidiary Issuer [Member]
Condensed Consolidating Statements of Operations [Abstract]
Revenue	1,636.1	1,573.0
Operating costs and expenses
Cost of sales	1,395.9	1,339.2
Selling, general and administrative	31.4	42.3
Unusual or Infrequent Item, or Both, Net of Insurance Proceeds		0.0		19.9
Research and development	9.2	8.8
Total operating costs and expenses	1,436.5	1,390.3
Operating income	199.6	182.7
Interest expense and financing fee amortization	(24.2)	(10.5)
Other expense, net	9.6	13.1
Income (loss) before income taxes and equity in net loss of affiliates and subsidiaries	185.0	185.3
Income tax provision	(33.7)	(53.0)
Income (loss) Before Equity In Net Income (Loss) Of Affiliates and Subsidiaries	151.3	132.3
Equity in net income of subsidiaries	19.4	17.7
Net income	170.7	150.0
Total other comprehensive income (loss)	(0.6)	13.8
Comprehensive Income, Net of Tax, Attributable to Parent	170.1	163.8
Assets
Cash and cash equivalents	642.0	$ 711.0	642.0	711.0	387.3	$ 700.0
Restricted Cash and Cash Equivalents	0.3		0.3		2.2
Accounts receivable, net	730.1		730.1		752.6
Unbilled Receivables, Current	467.7		467.7
Inventory	610.1		610.1		1,010.0
Other current assets	32.4		32.4		50.3
Total current assets	2,462.1		2,462.1		2,180.2
Property, plant and equipment, net	1,621.1		1,621.1		1,585.8
Unbilled Receivable, Non Current	53.5		53.5
Pension assets	351.3		351.3		327.2
Investment in subsidiary	686.5		686.5		704.4
Other assets	372.8		372.8		298.2
Total assets	5,547.3		5,547.3		5,095.8
Liabilities
Accounts payable	837.6		837.6		629.0
Accrued expenses	317.6		317.6		239.5
Profit sharing	38.4		38.4		103.4
Current portion of long-term debt	24.1		24.1		30.2
Advance payments, short-term	112.7		112.7		100.0
Billings in Excess of Cost, Current	153.5		153.5
Provision for Loss on Contracts	147.3		147.3
Deferred revenue and other deferred credits, short-term	6.6		6.6		63.6
Other current liabilities	68.8		68.8		324.3
Total current liabilities	1,706.6		1,706.6		1,490.0
Long-term debt	1,862.0		1,862.0		1,110.6
Advance payments, long-term	144.2		144.2		231.7
Pension/OPEB obligation	38.4		38.4		40.8
Billings in Excess of Cost, Noncurrent	353.8		353.8
Provision for Loss on Contacts, Non Current	72.6		72.6
Deferred grant income liability - non-current	5.8		5.8
Deferred revenue and other deferred credits	24.3		24.3		158.2
Other liabilities	303.2		303.2		343.1
Total equity	1,036.4		1,036.4		1,721.4
Total liabilities and equity	5,547.3		5,547.3		$ 5,095.8
Operating activities
Net cash (used in) operating activities			486.1	525.7
Investing activities
Purchase of property, plant and equipment			(142.9)	(122.1)
Net cash used in investing activities			(140.6)	(121.7)
Proceeds from Issuance of Senior Long-term Debt			1,300.0
Financing activities
Principal payments of debt			(4.3)	(0.7)
Collection on (repayment of) intercompany debt			49.7	65.1
Payments Related to Tax Withholding for Share-based Compensation			(15.5)	(13.8)
Proceeds from (Payments for) Other Financing Activities			(805.8)	(402.1)
Proceeds from subsidiary (payments to Parent) to pay dividends			(35.4)	(35.7)
Payments of Dividends			0.0	0.0
Net cash used in financing activities			(90.8)	(393.0)
Net increase in cash, cash equivalents, and restricted cash for the period			254.7	11.0
Cash and cash equivalents, beginning of period			365.1
Cash, cash equivalents, and restricted cash, end of period	$ 621.5		621.5
Spirit [Member]
Condensed Consolidating Statements of Operations [Abstract]
Revenue			4,831.3	4,701.1
Operating costs and expenses
Cost of sales			4,147.8	4,307.4
Selling, general and administrative			133.1	130.4
Research and development			27.8	19.4
Total operating costs and expenses			4,308.7	4,477.1
Operating income			522.6	224.0
Interest expense and financing fee amortization			(60.1)	(30.1)
Other expense, net			6.6	37.4
Income (loss) before income taxes and equity in net loss of affiliates and subsidiaries			469.1	231.3
Income tax provision			(88.6)	(70.2)
Income (loss) Before Equity In Net Income (Loss) Of Affiliates and Subsidiaries			380.5	161.1
Equity in net income of subsidiaries			64.8	76.8
Net income			445.3	237.9
Total other comprehensive income (loss)			(16.5)	33.6
Comprehensive Income, Net of Tax, Attributable to Parent			428.8	271.5
Investing activities
Proceeds from Sales of Assets, Investing Activities				0.4
Payments for (Proceeds from) Other Investing Activities			2.3	0.0
Financing activities
Repayments of Senior Debt			(256.3)	(12.5)
Early Repayment of Senior Debt			(300.0)
Debt issuance and financing costs			$ (23.2)	(0.9)
Proceeds from Other Debt				$ 7.6