Indebtedness - Principal Payments Due on Long-Term Borrowings (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Principal Payments Due on Long Term Borrowings [Abstract]
2018 (after September 30,)	$ 25.0
2019	450.0
2020	550.0
2021	800.0
2022	500.0
Thereafter	2,680.7
Total principal payment	5,005.7	$ 5,580.4
Commercial Paper [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2018 (after September 30,)	25.0
Total principal payment	25.0	130.0
2010 Senior Notes [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2020	500.0
Total principal payment	500.0	500.0
2012 Senior Notes [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2022	500.0
Total principal payment	500.0	500.0
2013 Senior Notes [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
Thereafter	500.0
Total principal payment	500.0	500.0
2014 Senior Notes (5-Year) [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2019	450.0
Total principal payment	450.0	450.0
2014 Senior Notes (30-Year) [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
Thereafter	600.0
Total principal payment	600.0	600.0
2015 Senior Notes [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
Thereafter	580.7
Total principal payment	580.7	600.4
2.75% 2017 Senior Notes, due 2021 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2021	500.0
Total principal payment	500.0	500.0
2017 Term Loan Facility, due 2020 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2020	50.0
Total principal payment	50.0	500.0
2.625% 2017 Notes, due 2023 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
Thereafter	500.0
Total principal payment	500.0	500.0
3.25% 2017 Notes, due 2028 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
Thereafter	500.0
Total principal payment	500.0	$ 500.0
3.25% 2018 Notes, due 2021 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2021	300.0
Total principal payment	$ 300.0

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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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