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	Oct. 18, 2013	Jun. 26, 2012	Mar. 25, 2008	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
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%
Five-Year Term Loan, due August 2021 \| Unsecured Debt
Debt Instrument [Line Items]
Credit facility, expiration date				Aug. 21, 2021			Aug. 21, 2021	Aug. 21, 2021
Debt instrument, term				5 years			5 years	5 years
Stated interest rate							0.00%	2.02%
Seven-Year Term Loan, due October 2020 \| Unsecured Debt
Debt Instrument [Line Items]
Debt instrument, term							7 years	7 years	7 years
Debt instrument, maturity date				Oct. 18, 2020			Oct. 18, 2020	Oct. 18, 2020	Oct. 18, 2020
Stated interest rate							0.00%	2.40%
6.50% Senior Notes due March 2018 \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, maturity date						Mar. 15, 2018	Mar. 15, 2018	Mar. 15, 2018
Stated interest rate						6.50%	6.50%	6.50%
2.45% Senior Notes, due December 2020 \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, maturity date	Dec. 15, 2020						Dec. 15, 2020
Stated interest rate	2.45%						2.45%	0.00%
Senior notes, discount							$ 0.5
3.90% Senior Notes, due June 2022 \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, maturity date					Jun. 15, 2022		Jun. 15, 2022	Jun. 15, 2022
Stated interest rate					3.90%		3.90%	3.90%
Senior notes, discount							$ 0.2	$ 0.2
4.50% Senior Notes, due November 2023 \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, maturity date			Nov. 01, 2023				Nov. 01, 2023	Nov. 01, 2023
Stated interest rate			4.50%				4.50%	4.50%
Senior notes, discount							$ 1.2	$ 1.4
3.65% Senior Notes, due September 2024 \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, maturity date		Sep. 15, 2024					Sep. 15, 2024	Sep. 15, 2024
Stated interest rate		3.65%					3.65%	3.65%
Senior notes, discount							$ 0.8	$ 0.9
3.40% Senior Notes, due December 2027 \| Senior Notes
Debt Instrument [Line Items]
Debt instrument, maturity date	Dec. 15, 2027						Dec. 15, 2027
Stated interest rate	3.40%						3.40%	0.00%
Senior notes, discount							$ 1.6