Summary of Total Indebtedness (Detail) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Principal Amount	$ 3,419.8	$ 3,393.1
Fair Value of Interest Rate Swap	32.3	11.7
Unamortized (Discount) Premium	(3.6)	(3.8)
Unamortized Debt Issuance Costs	(19.9)	(20.4)
Carrying Amount	3,428.6	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	22.1	9.4
Unamortized (Discount) Premium	(1.6)	(1.6)
Unamortized Debt Issuance Costs	(1.9)	(2.0)
Carrying Amount	518.6	505.8
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Unamortized (Discount) Premium	(2.7)	(2.8)
Unamortized Debt Issuance Costs	(2.4)	(2.5)
Carrying Amount	494.9	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	(3.0)	(3.1)
Carrying Amount	494.8	494.6
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	450.0	450.0
Fair Value of Interest Rate Swap	10.2	2.3
Unamortized (Discount) Premium	(0.5)	(0.5)
Unamortized Debt Issuance Costs	(2.2)	(2.4)
Carrying Amount	457.5	449.4
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	600.0	600.0
Unamortized (Discount) Premium	3.4	3.4
Unamortized Debt Issuance Costs	(6.1)	(6.2)
Carrying Amount	597.3	597.2
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	569.8	543.1
Unamortized Debt Issuance Costs	(4.2)	(4.0)
Carrying Amount	$ 565.6	$ 539.1

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

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