Income Taxes (Details) - USD ($) $ in Millions	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013	May. 31, 2012
Current:
Federal	$ 22.2	$ 11.4	$ (3.7)
State	0.4	0.4	(0.7)
Foreign	1.5	0.8	1.5
Total current	24.1	12.6	(2.9)
Deferred	(52.6)	19.0	28.3
Provision for income tax (benefit)	(28.5)	31.6	25.4
Income Tax Expense (Benefit), Total	(28.5)	31.6	$ 25.4
Income tax receivable	$ 2.1	$ 14.8
U.S. federal statutory income tax rate (as a percent)	35.00%	35.00%	35.00%
Reasons for difference between the provision for income taxes on pre-tax income from the amount computed by applying the U.S. federal statutory tax rate
Provision for income tax (benefit) at the federal statutory rate	$ (29.0)	$ 34.6	$ 26.0
State income taxes, net of federal benefit and refunds	0.2	0.2	0.4
Federal adjustments	0.5	(2.0)	0.3
Domestic Production Activities Deduction	(0.2)	(1.2)
State adjustments			(1.3)
Provision for income tax (benefit)	(28.5)	31.6	25.4
Income (Loss) from continuing operations before provision for income tax (benefit)	(83.0)	98.8	$ 74.2
Deferred tax assets-current attributable to:
Inventory costs	35.4	14.4
Advance billings	6.0	2.0
Employee benefits	3.5	7.9
Other		0.3
Deferred Tax Assets, Net of Valuation Allowance, Current, Total	58.3	24.6
Deferred tax assets-noncurrent attributable to:
Postretirement benefits	18.9	17.6
Advance billings	8.3	16.2
Foreign intangible assets		44.2
Other	3.9	6.7
Total deferred tax assets-noncurrent	31.1	84.7
Total deferred tax assets	89.4	109.3
Valuation allowance	(2.8)	(2.8)
Total deferred tax assets net of valuation allowance	86.6	106.5
Deferred tax liabilities attributable to:
Depreciation	(132.4)	(158.4)
Capitalized program development costs		(34.1)
Foreign intangible assets		(42.9)
Other	(0.5)	(3.2)
Total deferred tax liabilities	(132.9)	(238.6)
Net deferred tax liabilities	$ (46.3)	$ (132.1)
Effective income tax rate (as a percent)	34.30%	32.00%	34.20%
Decrease in deferred tax assets current	$ 5.5
Decrease in deferred tax liabilities noncurrent	5.5
Reduction in income tax expense related to tax provision to federal income tax return filing differences		$ 2.0
Domestic Production Activities Deduction tax benefit	0.2	1.2
Reduction in income tax expense related to reduction of the entity's state income tax rate				$ 1.3
Domestic
Reasons for difference between the provision for income taxes on pre-tax income from the amount computed by applying the U.S. federal statutory tax rate
Income (Loss) from continuing operations before provision for income tax (benefit)	(94.2)	81.9	$ 58.4
Foreign
Reasons for difference between the provision for income taxes on pre-tax income from the amount computed by applying the U.S. federal statutory tax rate
Income (Loss) from continuing operations before provision for income tax (benefit)	11.2	$ 16.9	$ 15.8
Precision Systems Manufacturing
Deferred tax assets-current attributable to:
Impairment of PSM assets	$ 13.4