Debt - Narrative (Details)		3 Months Ended
	Oct. 01, 2020 USD ($)	Dec. 31, 2020 USD ($) credit_facility	Dec. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)
Line Of Credit Facility [Line Items]
Proceeds from issuance of long-term debt		$ 597,390,000	$ 799,450,000
Maximum borrowing capacity		$ 2,200,000,000
Maximum debt-to-total-capitalization ratio		70.00%
Debt-to-total-capitalization ratio		0.43
Minimum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio		50.00%
Outstanding indebtedness		$ 15,000,000
Maximum
Line Of Credit Facility [Line Items]
Equity-to-total-capitalization ratio		60.00%
Outstanding indebtedness		$ 100,000,000
Revolving Credit Facility
Line Of Credit Facility [Line Items]
Number of credit facilities | credit_facility		4
Unsecured 1.50% Senior Notes, due 2031
Line Of Credit Facility [Line Items]
Debt instrument, face amount	$ 600,000,000
Interest rate	1.50%	1.50%
Proceeds from issuance of long-term debt	$ 592,300,000
Effective interest rate	1.71%
Five Year Unsecured Revolving Credit Agreement
Line Of Credit Facility [Line Items]
Outstanding commercial paper		$ 0		$ 0
Five Year Unsecured Revolving Credit Agreement | Commercial Paper
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		1,500,000,000
Accordion feature		250,000,000
Maximum borrowing capacity post accordion feature		$ 1,750,000,000
Debt agreement term		5 years
$600 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		$ 600,000,000
Debt agreement term		364 days
Outstanding borrowings		$ 0
$50 Million Bank Loan Agreement | Line of Credit
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		$ 50,000,000
Debt agreement term		364 days
Outstanding borrowings		$ 0
$50 Million Revolving Credit Facility | Revolving Credit Facility
Line Of Credit Facility [Line Items]
Maximum borrowing capacity		$ 50,000,000
Debt agreement term		364 days
Remaining borrowing capacity		$ 44,400,000
$50 Million Revolving Credit Facility | Line of Credit
Line Of Credit Facility [Line Items]
Outstanding borrowings		$ 0
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread		0.00%
LIBOR | Five Year Unsecured Revolving Credit Agreement | Commercial Paper | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread		1.25%
LIBOR | $600 Million Revolving Credit Facility | Revolving Credit Facility | Minimum
Line Of Credit Facility [Line Items]
Interest rate spread		0.00%
LIBOR | $600 Million Revolving Credit Facility | Revolving Credit Facility | Maximum
Line Of Credit Facility [Line Items]
Interest rate spread		1.25%