Debt (Detail) - USD ($)		Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Debt Instrument [Line Items]
Fair Value		$ 6,446,700,000	$ 6,793,500,000	$ 6,664,300,000
Debt, Long-term and Short-term, Combined Amount	[1]	5,397,500,000	5,396,100,000	5,394,700,000
3.75% Senior Notes due 2021 (issued: $500.0, August 2011)
Debt Instrument [Line Items]
Long-term Debt		499,900,000	499,800,000	499,600,000
Fair Value		$ 502,500,000	510,900,000	519,000,000.0
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		497,500,000	497,300,000	497,100,000
Fair Value		$ 529,900,000	541,100,000	533,700,000
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	296,800,000
Fair Value		$ 396,100,000	409,400,000	420,700,000
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		545,700,000	545,500,000	545,200,000
Fair Value		$ 631,500,000	660,400,000	654,200,000
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		496,300,000	496,100,000	496,000,000.0
Fair Value		$ 549,000,000.0	575,500,000	567,600,000
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	395,800,000
Fair Value		$ 553,700,000	582,000,000.0	569,600,000
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		$ 434,800,000	459,700,000	444,900,000
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		$ 458,100,000	481,000,000.0	471,400,000
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		841,800,000	841,700,000	841,600,000
Fair Value		$ 1,041,400,000	1,113,100,000	1,080,800,000
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		590,100,000	590,000,000.0	589,900,000
Fair Value		$ 747,000,000.0	806,700,000	773,800,000
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		490,700,000	490,600,000	490,500,000
Fair Value		$ 602,700,000	653,700,000	628,600,000
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.