Credit And Other Debt Agreements (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Debt Instrument [Line Items]
Debt installments, maturity date	Jul. 14, 2014	Jul. 14, 2014
Liabilities	$ 3,373.1	$ 3,239.9
Note Purchase Agreements [Member]
Debt Instrument [Line Items]
Amount payable to redeem the notes, percent of the principal amount	100.00%
Discount rate used to compute the remaining scheduled payments of principal and interest	U.S. Treasury yields plus 0.5%
Line of credit facility, fair value of amount outstanding	826.8
Long-term debt	725.0
Note Purchase Agreements [Member] | Senior Note, Series F [Member]
Debt Instrument [Line Items]
Notes issued and sold	50.0
Note payable, semi-annual payments of interest, fixed rate	3.99%
Debt installments, maturity date	Jul. 10, 2020
Note Purchase Agreements [Member] | Senior Note, Series A [Member]
Debt Instrument [Line Items]
Notes issued and sold	100.0
Note payable, semi-annual payments of interest, fixed rate	6.26%
Debt installments, maturity date	Aug. 03, 2014
Note Purchase Agreements [Member] | Senior Note, Series B [Member]
Debt Instrument [Line Items]
Notes issued and sold	300.0
Note payable, semi-annual payments of interest, fixed rate	6.44%
Debt installments, maturity date	Aug. 03, 2017
Credit Agreement [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum borrowing capacity	500.0
Line of credit facility, maximum amount outstanding during period	600.0
Fixed rate (alternative one) over LIBOR	1.45%
Fixed rate (alternative two) over LIBOR	1.65%
Fixed rate (alternative three) over LIBOR	1.85%
Fixed rate (alternative four) over LIBOR	2.00%
Annual facility fee of revolving credit facility (alternative one)	0.30%
Annual facility fee of revolving credit facility (alternative two)	0.35%
Annual facility fee of revolving credit facility (alternative three)	0.40%
Annual facility fee of revolving credit facility (alternative four)	0.50%
Credit Agreement [Member] | Standby Letters of Credit [Member]
Debt Instrument [Line Items]
Long-term debt, gross	75.0
Line of credit facility, amount outstanding	15.9
Liabilities	8.6
Line of credit facility, fair value of amount outstanding	0
Line of credit facility, remaining borrowing capacity	484.1
Line of credit facility, increase, additional borrowings	59.1
Credit Agreement [Member] | Swing Loan [Member]
Debt Instrument [Line Items]
Long-term debt, gross	50.0
First Installment [Member] | Note Purchase Agreements [Member] | Senior Note, Series C [Member]
Debt Instrument [Line Items]
Notes issued and sold	150.0
Note payable, semi-annual payments of interest, fixed rate	5.85%
Debt installments, maturity date	Nov. 30, 2016
First Installment [Member] | Note Purchase Agreements [Member] | Senior Note, Series D [Member]
Debt Instrument [Line Items]
Notes issued and sold	75.0
Note payable, semi-annual payments of interest, fixed rate	5.18%
Debt installments, maturity date	Feb. 10, 2021
Second Installment [Member] | Note Purchase Agreements [Member] | Senior Note, Series C [Member]
Debt Instrument [Line Items]
Debt installments, maturity date	Nov. 30, 2018
Second Installment [Member] | Note Purchase Agreements [Member] | Senior Note, Series E [Member]
Debt Instrument [Line Items]
Notes issued and sold	$ 50.0
Note payable, semi-annual payments of interest, fixed rate	5.49%
Debt installments, maturity date	Feb. 23, 2023
Third Installment [Member] | Note Purchase Agreements [Member] | Senior Note, Series C [Member]
Debt Instrument [Line Items]
Debt installments, maturity date	Nov. 30, 2019