Debt (Detail) - USD ($)		Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Debt Instrument [Line Items]
Fair Value		$ 5,881,700,000	$ 6,793,500,000	$ 6,687,400,000
Debt, Long-term and Short-term, Combined Amount	[1]	4,898,200,000	5,396,100,000	5,395,400,000
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
Debt Instrument [Line Items]
Long-term Debt		0	499,800,000	499,700,000
Fair Value		$ 0	510,900,000	515,300,000
Debt, stated interest rate		3.75%
Debt, face amount		$ 500,000,000.0	500,000,000.0	500,000,000.0
2.45% Senior Notes due 2027 (issued: $500.0, August 2016)
Debt Instrument [Line Items]
Long-term Debt		497,600,000	497,300,000	497,200,000
Fair Value		$ 530,600,000	541,100,000	532,000,000.0
Debt, stated interest rate		2.45%
Debt, face amount		$ 500,000,000.0	500,000,000.0	500,000,000.0
6 5/8% Senior Notes due 2029 (issued: $300.0, March 1999)
Debt Instrument [Line Items]
Long-term Debt		297,100,000	296,900,000	296,800,000
Fair Value		$ 390,900,000	409,400,000	411,400,000
Debt, stated interest rate		6.625%
Debt, face amount		$ 300,000,000.0	300,000,000.0	300,000,000.0
4.00% Senior Notes due 2029 (issued: $550.0, October 2018)
Debt Instrument [Line Items]
Long-term Debt		545,800,000	545,500,000	545,300,000
Fair Value		$ 627,900,000	660,400,000	662,500,000
Debt, stated interest rate		4.00%
Debt, face amount		$ 550,000,000.0	550,000,000.0	550,000,000.0
3.20% Senior Notes due 2030 (issued: $500.0, March 2020)
Debt Instrument [Line Items]
Long-term Debt		496,400,000	496,100,000	496,000,000.0
Fair Value		$ 543,200,000	575,500,000	572,400,000
Debt, stated interest rate		3.20%
Debt, face amount		$ 500,000,000.0	500,000,000.0	500,000,000.0
6.25% Senior Notes due 2032 (issued: $400.0, November 2002)
Debt Instrument [Line Items]
Long-term Debt		396,100,000	396,000,000.0	395,900,000
Fair Value		$ 545,500,000	582,000,000.0	587,200,000
Debt, stated interest rate		6.25%
Debt, face amount		$ 400,000,000.0	400,000,000.0	400,000,000.0
4.35% Senior Notes due 2044 (issued: $350.0, April 2014)
Debt Instrument [Line Items]
Long-term Debt		346,800,000	346,700,000	346,700,000
Fair Value		$ 433,600,000	459,700,000	450,500,000
Debt, stated interest rate		4.35%
Debt, face amount		$ 350,000,000.0	350,000,000.0	350,000,000.0
3.70% Senior Notes due 2045 (issued: $400.0, January 2015)
Debt Instrument [Line Items]
Long-term Debt		395,600,000	395,500,000	395,500,000
Fair Value		$ 456,200,000	481,000,000.0	473,500,000
Debt, stated interest rate		3.70%
Debt, face amount		$ 400,000,000.0	400,000,000.0	400,000,000.0
4.125% Senior Notes due 2047 (issued: $850.0, April 2017)
Debt Instrument [Line Items]
Long-term Debt		841,900,000	841,700,000	841,700,000
Fair Value		$ 1,018,500,000	1,113,100,000	1,078,500,000
Debt, stated interest rate		4.125%
Debt, face amount		$ 850,000,000.0	850,000,000.0	850,000,000.0
4.20% Senior Notes due 2048 (issued: $600.0, March 2018)
Debt Instrument [Line Items]
Long-term Debt		590,200,000	590,000,000.0	590,000,000.0
Fair Value		$ 736,300,000	806,700,000	775,300,000
Debt, stated interest rate		4.20%
Debt, face amount		$ 600,000,000.0	600,000,000.0	600,000,000.0
3.95% Senior Notes due 2050 (issued: $500.0, March 2020)
Debt Instrument [Line Items]
Long-term Debt		490,700,000	490,600,000	490,600,000
Fair Value		$ 599,000,000.0	653,700,000	628,800,000
Debt, stated interest rate		3.95%
Debt, face amount		$ 500,000,000.0	$ 500,000,000.0	$ 500,000,000.0

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