Debt (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended			3 Months Ended			6 Months Ended	1 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jun. 30, 2012 8.5 % Notes Due July 1,2017	Jun. 30, 2012 4.6% Notes Due July 1, 2013	Mar. 31, 2012 4.6% Notes Due July 1, 2013	Jun. 30, 2012 4.6% Notes Due July 1, 2013	Apr. 30, 2012 Unsecured Five-Year Revolving Credit Facility	Jun. 30, 2012 Unsecured Five-Year Revolving Credit Facility
Debt Instrument [Line Items]
Interest rate of debt instrument, percentage						8.95%	4.60%	4.60%	4.60%
Maturity date of debt instrument						Jul. 01, 2017	Jul. 01, 2013	Jul. 01, 2013	Jul. 01, 2013
Carrying value of notes repurchased						$ 0.6	$ 3.8	$ 1.6
Price of notes repurchased						0.8	4.1	1.7
Loss on early extinguishment of debt	(0.2)	(0.2)	(0.2)	(0.3)		0.2	0.2	0.1
Current borrowing of unsecured five-year revolving credit facility										210.0	170.0
Remaining borrowing of unsecured five-year revolving credit facility											230.0
Total borrowing of unsecured five-year revolving credit facility											400.0
Fair value of our long-term debt	$ 900.4		$ 900.4		$ 715.4
Weighted-average interest rate	5.68%	6.90%	5.68%	6.90%
Weighted-average interest rate on short-term borrowings	0.78%	1.54%	0.78%	1.54%