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

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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The cash inflow from a borrowing supported by a written promise to pay an obligation.

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Notes Payable - Note Issuances (Details) - USD ($)	1 Months Ended		9 Months Ended
Notes Payable - Note Issuances (Details) - USD ($)	Apr. 30, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Sep. 30, 2014
Debt
Proceeds from issuance of debt			$ 497,500,000	$ 711,812,000
Notes Payable \| 3.875% notes, issued in April 2018 and due in April 2025
Debt
Face amount of notes	$ 500,000,000.0
Stated interest rate (as a percent)	3.875%		3.875%		3.875%
Effective yield (as a percent)	3.957%
Public offering price	99.50%
Proceeds from issuance of debt	$ 493,100,000
Notes Payable \| 4.650% notes, issued in March 2017 and due in March 2047
Debt
Face amount of notes		$ 300,000,000.0
Effective yield (as a percent)		4.65%
Public offering price		99.97%
Notes Payable \| 4.125% notes, issued in March 2017 and due in October 2026
Debt
Face amount of notes		$ 400,000,000.0
Stated interest rate (as a percent)		4.125%
Effective yield (as a percent)		3.75%
Public offering price		102.98%
Notes Payable \| 4.125% notes, issued in September 2014 and due in October 2026
Debt
Face amount of notes						$ 250,000,000.0
Notes Payable \| Notes issued in March 2017
Debt
Proceeds from issuance of debt		$ 705,200,000