Debt (Detail) - USD ($) $ in Millions		Jun. 30, 2019	Dec. 31, 2018	Jun. 30, 2018
Debt Instrument [Line Items]
Fair Value		$ 4,955.2	$ 4,532.3	$ 3,959.0
Debt, Long-term and Short-term, Combined Amount	[1]	4,406.0	4,404.9	3,859.5
3.75% Senior Notes due 2021
Debt Instrument [Line Items]
Long-term Debt		499.2	499.1	498.9
Fair Value		516.5	506.5	506.4
2.45% Senior Notes due 2027
Debt Instrument [Line Items]
Long-term Debt		496.7	496.5	496.3
Fair Value		494.7	455.5	451.3
6 5/8% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		296.5	296.4	296.2
Fair Value		385.7	368.5	364.4
4.00% Senior Notes due 2029
Debt Instrument [Line Items]
Long-term Debt		544.8	544.5	0.0
Fair Value		608.9	562.4	0.0
6.25% Senior Notes due 2032
Debt Instrument [Line Items]
Long-term Debt		395.6	395.5	395.4
Fair Value		531.3	496.6	492.5
4.35% Senior Notes due 2044
Debt Instrument [Line Items]
Long-term Debt		346.6	346.6	346.6
Fair Value		392.5	350.2	356.7
3.70% Senior Notes due 2045
Debt Instrument [Line Items]
Long-term Debt		395.4	395.3	395.3
Fair Value		411.1	366.7	363.9
4.125% Senior Notes due 2047
Debt Instrument [Line Items]
Long-term Debt		841.5	841.4	841.3
Fair Value		943.0	831.9	831.5
4.20% Senior Notes due 2048
Debt Instrument [Line Items]
Long-term Debt		589.7	589.6	589.5
Fair Value		$ 671.5	$ 594.0	$ 592.3

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