Indebtedness - Principal Payments Due on Long-Term Borrowings (Detail) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Principal Payments Due on Long Term Borrowings [Abstract]
2019 (after March 31,)	$ 320.0
2020	500.0
2021	800.0
2022	500.0
2023	500.0
Thereafter	2,961.4
Total principal payment	5,581.4	$ 5,721.6
Commercial Paper [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2019 (after March 31,)	320.0
Total principal payment	320.0
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]
Total principal payment		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	561.4
Total principal payment	561.4	571.6
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
2.625% 2017 Notes, due 2023 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
2023	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	300.0
4.25% 2018 Notes, due 2029 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
Thereafter	400.0
Total principal payment	400.0	400.0
4.875% 2018 Notes, due 2048 [Member]
Principal Payments Due on Long Term Borrowings [Abstract]
Thereafter	400.0
Total principal payment	$ 400.0	$ 400.0

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

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