Financial Instruments and Risk Management (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 31, 2013	Mar. 31, 2012
Total exposure to floating rate interest rates		1.00%
Unrealized gains on interest-rate swap agreements	$ 6.0	$ 10.9
Gain (loss) on extinguishment of debt	0.0	1.1	$ (5.5)
Loss written-off resulting from non-performance of counterparties	0.2
Accelerated deferred loss - treasury lock agreements		2.6
Foreign Exchange Contract [Member]
Notional amounts of derivative contracts	39.0
Gain (Loss) in other expense from undesignated foreign currency exchange contracts	3.0	(13.5)
2007 Lock Agreement [Member]
Notional amounts of derivative contracts			500.0
Losses on 2007 Lock Agreement			38.0
2007 Lock Agreement [Member] | 2013 Notes [Member]
Principal amount of debt			250.0
2007 Lock Agreement [Member] | 2018 Notes [Member]
Principal amount of debt			250.0
Intercompany Loans [Member]
Gain (Loss) related to intercompany transactions for foreign currency change	5.2	10.4
Losses Amortized to Interest Expense in Five Years [Member] | 2007 Lock Agreement [Member]
Losses on 2007 Lock Agreement			19.2
Losses Amortized to Interest Expense in Ten Years [Member] | 2007 Lock Agreement [Member]
Losses on 2007 Lock Agreement			$ 18.8
January 2013 Interest-Rate Swap Termination [Member]
Notional Amount Related to Discontinuation of Interest Rate Fair Value Hedge				$ 1,000.0
Unrealized gains on interest-rate swap agreements	0.0			$ 90.4
Amortization of Deferred Hedge Gains		35.4
Gain (loss) on extinguishment of debt		23.6
March 2012 Interest-Rate Swap Termination [Member]
Notional Amount Related to Discontinuation of Interest Rate Fair Value Hedge					$ 350.0
Unrealized gains on interest-rate swap agreements	6.0				$ 46.1
Amortization of Deferred Hedge Gains	$ 4.9	11.9
Gain (loss) on extinguishment of debt		$ 5.8