Debt (Detail) - USD ($) $ in Millions		Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Debt Instrument [Line Items]
Fair Value		$ 5,113.3	$ 4,532.3	$ 3,926.9
Debt, Long-term and Short-term, Combined Amount	[1]	4,406.5	4,404.9	3,859.9
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt		499.3	499.1	499.0
Fair Value		516.5	506.5	503.9
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Long-term Debt		496.8	496.5	496.4
Fair Value		502.1	455.5	453.3
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		296.5	296.4	296.3
Fair Value		392.2	368.5	360.3
4.00% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		544.9	544.5	0.0
Fair Value		617.0	562.4	0.0
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Long-term Debt		395.7	395.5	395.4
Fair Value		547.2	496.6	481.8
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Long-term Debt		346.6	346.6	346.6
Fair Value		414.7	350.2	349.7
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Long-term Debt		395.4	395.3	395.3
Fair Value		431.8	366.7	359.1
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Long-term Debt		841.5	841.4	841.3
Fair Value		987.4	831.9	828.4
4.20% Senior Notes due 2048
Debt Instrument [Line Items]
Long-term Debt		589.8	589.6	589.6
Fair Value		$ 704.4	$ 594.0	$ 590.4

[1] Consists of long-term debt. See Note 4 – Debt for further discussion.