Operating Segments and Related Disclosures - Selected Financial Information for Each Reportable Segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Sales:
Net favorable effect of the changes in contract estimates on net sales	$ 556.5	$ 527.7	$ 512.4	$ 476.1	$ 523.0	$ 481.8	$ 485.0	$ 491.7	$ 2,072.7	$ 1,981.5	$ 1,895.9
Segment Performance:
Environmental remediation provision adjustments									(4.3)	(2.1)	36.9
Reconciliation of segment performance to income before income taxes:
Interest expense									(30.1)	(35.7)	(34.4)
Interest income									6.3	15.5	10.0
Stock-based compensation									(31.4)	(27.3)	(20.5)
Income before income taxes									180.2	191.9	188.6
Capital Expenditures									54.6	42.9	43.2
Depreciation and amortization									65.3	74.5	72.3
Aerospace and Defense
Reconciliation of segment performance to income before income taxes:
Capital Expenditures									54.6	42.9	42.2
Depreciation and amortization									64.2	72.9	71.1
Real Estate Segment
Reconciliation of segment performance to income before income taxes:
Capital Expenditures									0.0	0.0	0.0
Depreciation and amortization									0.8	1.2	0.9
Operating Segments
Net Sales:
Net favorable effect of the changes in contract estimates on net sales									2,072.7	1,981.5	1,895.9
Segment Performance:
Segment performance									273.8	271.4	267.4
Reconciliation of segment performance to income before income taxes:
Segment performance									273.8	271.4	267.4
Operating Segments | Aerospace and Defense
Net Sales:
Net favorable effect of the changes in contract estimates on net sales									2,069.4	1,974.0	1,888.1
Segment Performance:
Aerospace and Defense									264.2	249.1	233.4
Environmental remediation provision adjustments									(2.7)	(1.9)	37.2
Retirement benefits, net									14.7	22.4	(6.0)
Unusual items									(0.6)	(0.3)	0.0
Segment performance									275.6	269.3	264.6
Reconciliation of segment performance to income before income taxes:
Segment performance									275.6	269.3	264.6
Unusual items									(0.6)	(0.3)	0.0
Operating Segments | Real Estate Segment
Net Sales:
Net favorable effect of the changes in contract estimates on net sales									3.3	7.5	7.8
Segment Performance:
Segment performance									(1.8)	2.1	2.8
Reconciliation of segment performance to income before income taxes:
Segment performance									(1.8)	2.1	2.8
Segment Reconciling Items
Segment Performance:
Unusual items									(7.5)	0.0	(0.2)
Reconciliation of segment performance to income before income taxes:
Interest expense									(30.1)	(35.7)	(34.4)
Interest income									6.3	15.5	10.0
Stock-based compensation									(31.4)	(27.3)	(20.5)
Unusual items									(7.5)	0.0	(0.2)
Corporate
Segment Performance:
Retirement benefits, net									(7.5)	(7.2)	(13.4)
Reconciliation of segment performance to income before income taxes:
Corporate and other									(23.4)	(24.8)	(20.3)
Capital Expenditures									0.0	0.0	1.0
Depreciation and amortization									$ 0.3	$ 0.4	$ 0.3