Contractual interest rate for funds borrowed, under the debt agreement.

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Debt - Summary of Debt (Parenthetical) (Details) - USD ($) $ in Millions						12 Months Ended
	Nov. 21, 2019	Dec. 13, 2017	Sep. 05, 2014	Oct. 22, 2013	Jun. 26, 2012	Dec. 31, 2019	Dec. 31, 2018	Dec. 23, 2019	Dec. 06, 2019	Dec. 31, 2017
Revolving Credit Facility, due August 2021 \| Line of Credit
Debt Instrument [Line Items]
Credit facility, expiration date						Aug. 21, 2021	Aug. 21, 2021
Stated interest rate						0.00%	0.00%
2.45% Senior Notes, due December 2020
Debt Instrument [Line Items]
Stated interest rate						2.45%				2.45%
2.45% Senior Notes, due December 2020 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate	2.45%	2.45%					2.45%		2.45%
Senior notes, discount							$ 0.4
Debt instrument, maturity date	Dec. 15, 2020	Dec. 15, 2020					Dec. 15, 2020
3.90% Senior Notes, due June 2022
Debt Instrument [Line Items]
Stated interest rate						3.90%
3.90% Senior Notes, due June 2022 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate	3.90%				3.90%		3.90%	3.90%
Senior notes, discount							$ 0.1
Debt instrument, maturity date	Jun. 15, 2022				Jun. 15, 2022		Jun. 15, 2022
4.50% Senior Notes, due November 2023 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate				4.50%		4.50%	4.50%
Senior notes, discount						$ 0.8	$ 1.0
Debt instrument, maturity date				Nov. 01, 2023		Nov. 01, 2023	Nov. 01, 2023
3.65% Senior Notes, due September 2024 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate			3.65%			3.65%	3.65%
Senior notes, discount						$ 0.6	$ 0.7
Debt instrument, maturity date			Sep. 15, 2024			Sep. 15, 2024	Sep. 15, 2024
3.40% Senior Notes, due December 2027
Debt Instrument [Line Items]
Stated interest rate										3.40%
3.40% Senior Notes, due December 2027 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate		3.40%				3.40%	3.40%
Senior notes, discount						$ 1.3	$ 1.5
Debt instrument, maturity date		Dec. 15, 2027				Dec. 15, 2027	Dec. 15, 2027
3.00% Senior Notes, due December 2029
Debt Instrument [Line Items]
Stated interest rate						3.00%
3.00% Senior Notes, due December 2029 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate	3.00%					3.00%
Senior notes, discount						$ 0.7
Debt instrument, maturity date	Dec. 15, 2029					Dec. 15, 2029
4.05% Senior Notes, due December 2049
Debt Instrument [Line Items]
Stated interest rate						4.05%
4.05% Senior Notes, due December 2049 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate	4.05%					4.05%
Senior notes, discount						$ 3.5
Debt instrument, maturity date	Dec. 15, 2049					Dec. 15, 2049