Debt (Detail) - USD ($)		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Debt Instrument [Line Items]
Fair Value		$ 6,195,200,000	$ 6,793,500,000	$ 6,108,600,000
Debt, Long-term and Short-term, Combined Amount	[1]	5,396,800,000	5,396,100,000	5,394,000,000.0
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
Debt Instrument [Line Items]
Long-term Debt		499,900,000	499,800,000	499,500,000
Fair Value		$ 506,700,000	510,900,000	510,700,000
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		497,400,000	497,300,000	497,000,000.0
Fair Value		$ 526,800,000	541,100,000	488,200,000
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		297,000,000.0	296,900,000	296,700,000
Fair Value		$ 386,100,000	409,400,000	395,300,000
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		545,600,000	545,500,000	545,100,000
Fair Value		$ 623,700,000	660,400,000	605,700,000
Debt, stated interest rate		4.00%
Debt, face amount		$ 550,000,000.0	550,000,000.0	550,000,000.0
Senior Notes Three Point Two Zero Percent Due Twenty Thirty [Member]
Debt Instrument [Line Items]
Long-term Debt		496,200,000	496,100,000	495,800,000
Fair Value		$ 536,700,000	575,500,000	543,200,000
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		396,000,000.0	396,000,000.0	395,800,000
Fair Value		$ 531,400,000	582,000,000.0	539,600,000
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		$ 413,900,000	459,700,000	391,700,000
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	395,400,000
Fair Value		$ 430,200,000	481,000,000.0	406,200,000
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		841,700,000	841,700,000	841,600,000
Fair Value		$ 977,100,000	1,113,100,000	991,900,000
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		590,000,000.0	590,000,000.0	589,900,000
Fair Value		$ 698,700,000	806,700,000	675,800,000
Debt, stated interest rate		4.20%
Debt, face amount		$ 600,000,000.0	600,000,000.0	600,000,000.0
Senior Notes Three Point Nine Five Percent Due Twenty Fifty [Member]
Debt Instrument [Line Items]
Long-term Debt		490,600,000	490,600,000	490,500,000
Fair Value		$ 563,900,000	653,700,000	560,300,000
Debt, stated interest rate		3.95%
Debt, face amount		$ 500,000,000.0	$ 500,000,000.0	$ 500,000,000.0

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