Summary of Total Indebtedness (Detail) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal Amount	$ 4,920.3	$ 3,377.4
Fair Value of Interest Rate Swap	6.3	6.2
Unamortized (Discount) Premium	(10.3)	(2.9)
Unamortized Debt Issuance Costs	(29.2)	(17.7)
Carrying amount	4,887.1	3,363.0
Current portion of long-term debt		(300.0)
Total long-term debt	4,887.1	3,063.0
Series 2007-1 Notes [Member]
Debt Instrument [Line Items]
Principal Amount		300.0
Carrying amount		300.0
2010 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Fair Value of Interest Rate Swap	5.6	5.5
Unamortized (Discount) Premium	(1.2)	(1.3)
Unamortized Debt Issuance Costs	(1.4)	(1.6)
Carrying amount	503.0	502.6
2012 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Fair Value of Interest Rate Swap	0.1	(0.2)
Unamortized (Discount) Premium	(2.2)	(2.4)
Unamortized Debt Issuance Costs	(1.9)	(2.1)
Carrying amount	496.0	495.3
2013 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0	500.0
Unamortized (Discount) Premium	(1.9)	(2.1)
Unamortized Debt Issuance Costs	(2.5)	(2.7)
Carrying amount	495.6	495.2
2014 Senior Notes (5-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	450.0	450.0
Fair Value of Interest Rate Swap	0.6	0.9
Unamortized (Discount) Premium	(0.3)	(0.4)
Unamortized Debt Issuance Costs	(1.4)	(1.7)
Carrying amount	448.9	448.8
2014 Senior Notes (30-Year) [Member]
Debt Instrument [Line Items]
Principal Amount	600.0	600.0
Unamortized (Discount) Premium	3.3	3.3
Unamortized Debt Issuance Costs	(5.8)	(5.9)
Carrying amount	597.5	597.4
2015 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	570.3	527.4
Unamortized Debt Issuance Costs	(3.6)	(3.7)
Carrying amount	566.7	$ 523.7
2017 Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	500.0
Unamortized (Discount) Premium	(1.4)
Unamortized Debt Issuance Costs	(3.7)
Carrying amount	494.9
2017 Floating Rate Senior Notes [Member]
Debt Instrument [Line Items]
Principal Amount	300.0
Unamortized Debt Issuance Costs	(1.0)
Carrying amount	299.0
2.65% 2017 Private Placement Notes, due 2023 [Member]
Debt Instrument [Line Items]
Principal Amount	500.0
Unamortized (Discount) Premium	(1.2)
Unamortized Debt Issuance Costs	(3.8)
Carrying amount	495.0
3.25% 2017 Private Placement Notes, due 2028 [Member]
Debt Instrument [Line Items]
Principal Amount	500.0
Unamortized (Discount) Premium	(5.4)
Unamortized Debt Issuance Costs	(4.1)
Carrying amount	$ 490.5

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

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