Stockholders' Equity (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI	$ (2.0)		$ 0.1	$ 24.6	$ 6.2
Common shares repurchases				3,426.1
Accelerated Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Common shares repurchases		$ 512.0
Number of shares repurchased (in shares)	1,452,119	9,253,668
Purchase price per share (in dollars per share)	$ 47.82
Common Share Repurchases
Equity, Class of Treasury Stock [Line Items]
Common shares repurchases	$ 119.3	$ 3,300.0
Equity Tender Offer
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)		57,300,000
Purchase price per share (in dollars per share)		$ 48
Open Market Repurchases
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	2,660,893	818,071
Purchase price per share (in dollars per share)	$ 44.82	$ 46.45
Foreign currency translation adjustments
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI				26.2	4.7
Other comprehensive income, tax	$ 9.2		(1.4)	26.3	13.3
Foreign currency translation adjustments | Other Income
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI	0.0		0.0	26.2	4.7
Foreign currency translation adjustments | Other Income | Commercial Air | Discontinued Operations, Disposed of by Sale
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI		$ 16.7
Changes in benefit plan net gain/(loss) and prior service (cost)/credit losses
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI				0.7	1.5
Other comprehensive income, tax	0.0		0.0	(0.6)	0.0
Changes in benefit plan net gain/(loss) and prior service (cost)/credit losses | Operating Expenses
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI	0.1		0.1	0.7	1.5
Unrealized net gains on available for sale securities
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI				(2.3)	0.0
Other comprehensive income, tax	(2.3)		(3.3)	(6.5)	(12.4)
Unrealized net gains on available for sale securities | Other Income
Equity, Class of Treasury Stock [Line Items]
Amounts reclassified from AOCI	$ (2.1)		$ 0.0	$ (2.3)	$ 0.0