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Notes Payable - Components of Notes Payable (Parenthetical) (Detail) | 12 Months Ended |  |  |
--- | --- | --- | --- | ---
Dec. 31, 2017 | Nov. 05, 2015 | Mar. 17, 2014 | Apr. 21, 2011
Series A senior notes [Member] | 4.03% Series A senior notes due in 2018 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior Notes, interest rate | 4.03% |  |  | 4.03%
Senior Notes, year of maturity | 2018 |  |  |
Series B senior notes [Member] | 3.68% Series B senior notes due in 2021 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior Notes, interest rate | 3.68% |  | 3.68% |
Senior Notes, year of maturity | 2021 |  |  |
Series C senior notes [Member] | 3.84% Series C senior notes due in 2022 [Member] |  |  |  |
Debt Instrument [Line Items] |  |  |  |
Senior Notes, interest rate | 3.84% | 3.84% |  |
Senior Notes, year of maturity | 2022 |  |  |

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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.22(a)(1)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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