Summary of Total Indebtedness (Detail) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Principal Amount	$ 3,405.5	$ 3,393.1
Fair Value of Interest Rate Swap	37.5	11.7
Unamortized (Discount) Premium	(3.5)	(3.8)
Unamortized Debt Issuance Costs	(19.1)	(20.4)
Carrying Amount	3,420.4	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	25.5	9.4
Unamortized (Discount) Premium	(1.5)	(1.6)
Unamortized Debt Issuance Costs	(1.8)	(2.0)
Carrying Amount	522.2	505.8
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Unamortized (Discount) Premium	(2.6)	(2.8)
Unamortized Debt Issuance Costs	(2.3)	(2.5)
Carrying Amount	495.1	494.7
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Unamortized (Discount) Premium	(2.2)	(2.3)
Unamortized Debt Issuance Costs	(2.9)	(3.1)
Carrying Amount	494.9	494.6
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	450.0	450.0
Fair Value of Interest Rate Swap	12.0	2.3
Unamortized (Discount) Premium	(0.5)	(0.5)
Unamortized Debt Issuance Costs	(2.1)	(2.4)
Carrying Amount	459.4	449.4
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	600.0	600.0
Fair Value of Interest Rate Swap
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	555.5	543.1
Unamortized Debt Issuance Costs	(3.9)	(4.0)
Carrying Amount	$ 551.6	$ 539.1

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

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