Summary of Total Indebtedness (Detail) - USD ($) $ in Millions	Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Principal Amount	$ 3,411.9	$ 3,393.1
Fair Value of Interest Rate Swap	28.0	11.7
Unamortized (Discount) Premium	(3.2)	(3.8)
Unamortized Debt Issuance Costs	(18.6)	(20.4)
Carrying amount	3,418.1	3,380.6
Current portion of long-term debt	(299.9)
Total long-term debt	3,118.2	3,380.6
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Principal Amount	300.0	300.0
Unamortized Debt Issuance Costs	(0.1)	(0.2)
Carrying amount	299.9	299.8
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Fair Value of Interest Rate Swap	20.0	9.4
Unamortized (Discount) Premium	(1.4)	(1.6)
Unamortized Debt Issuance Costs	(1.7)	(2.0)
Carrying amount	516.9	505.8
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Unamortized (Discount) Premium	(2.5)	(2.8)
Unamortized Debt Issuance Costs	(2.2)	(2.5)
Carrying amount	495.3	494.7
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Unamortized (Discount) Premium	(2.1)	(2.3)
Unamortized Debt Issuance Costs	(2.8)	(3.1)
Carrying amount	495.1	494.6
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	450.0	450.0
Fair Value of Interest Rate Swap	8.0	2.3
Unamortized (Discount) Premium	(0.5)	(0.5)
Unamortized Debt Issuance Costs	(1.9)	(2.4)
Carrying amount	455.6	449.4
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	600.0	600.0
Unamortized (Discount) Premium	3.3	3.4
Unamortized Debt Issuance Costs	(6.0)	(6.2)
Carrying amount	597.3	597.2
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	561.9	543.1
Unamortized Debt Issuance Costs	(3.9)	(4.0)
Carrying amount	$ 558.0	$ 539.1

Face (par) amount of debt instrument at time of issuance.

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