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
	Dec. 13, 2017	Sep. 05, 2014	Oct. 22, 2013	Jun. 26, 2012	Dec. 31, 2018	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%
6.50% Senior Notes due March 2018 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate					6.50%	6.50%
Debt instrument, maturity date					Mar. 15, 2018	Mar. 15, 2018
2.45% Senior Notes, due December 2020
Debt Instrument [Line Items]
Stated interest rate						2.45%
2.45% Senior Notes, due December 2020 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate	2.45%				2.45%	2.45%
Debt instrument, maturity date	Dec. 15, 2020				Dec. 15, 2020	Dec. 15, 2020
Senior notes, discount					$ 0.4	$ 0.5
3.90% Senior Notes, due June 2022 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate				3.90%	3.90%	3.90%
Debt instrument, maturity date				Jun. 15, 2022	Jun. 15, 2022	Jun. 15, 2022
Senior notes, discount					$ 0.1	$ 0.2
4.50% Senior Notes, due November 2023 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate			4.50%		4.50%	4.50%
Debt instrument, maturity date			Nov. 01, 2023		Nov. 01, 2023	Nov. 01, 2023
Senior notes, discount					$ 1.0	$ 1.2
3.65% Senior Notes, due September 2024 \| Senior Notes
Debt Instrument [Line Items]
Stated interest rate		3.65%			3.65%	3.65%
Debt instrument, maturity date		Sep. 15, 2024			Sep. 15, 2024	Sep. 15, 2024
Senior notes, discount					$ 0.7	$ 0.8
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%
Debt instrument, maturity date	Dec. 15, 2027				Dec. 15, 2027	Dec. 15, 2027
Senior notes, discount					$ 1.5	$ 1.6