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Debt - Summary of Outstanding Debt (Parenthetical) (Detail) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Senior Unsecured Notes Series D [Member] |  |
Debt Instrument [Line Items] |  |
Stated interest rate on debt instrument |  | 3.22%
Senior Unsecured Notes Series B [Member] |  |
Debt Instrument [Line Items] |  |
Stated interest rate on debt instrument | 5.00% | 5.00%
Senior Unsecured Notes Series E [Member] |  |
Debt Instrument [Line Items] |  |
Stated interest rate on debt instrument | 3.97% | 3.97%
Senior Unsecured Notes Series F [Member] |  |
Debt Instrument [Line Items] |  |
Stated interest rate on debt instrument | 3.40% | 3.40%
Senior Unsecured Notes Series G [Member] |  |
Debt Instrument [Line Items] |  |
Stated interest rate on debt instrument | 3.92% | 3.92%
Senior Unsecured Notes Series H [Member] |  |
Debt Instrument [Line Items] |  |
Interest rate terms on debt | Series H senior unsecured notes bear interest at a 3 month LIBOR for that floating rate interest period plus 1.25%. | Series H senior unsecured notes bear interest at a 3 month LIBOR for that floating rate interest period plus 1.25%.
Senior Unsecured Notes Series H [Member] | LIBOR [Member] |  |
Debt Instrument [Line Items] |  |
Interest rate margin | 1.25% | 1.25%
Senior Unsecured Notes Series I [Member] |  |
Debt Instrument [Line Items] |  |
Stated interest rate on debt instrument | 3.13% | 3.13%
Senior Unsecured Notes Series J [Member] |  |
Debt Instrument [Line Items] |  |
Interest rate terms on debt |  | Series J senior unsecured notes bore interest at a 3-month LIBOR for that floating rate interest period plus 1.45%.
Senior Unsecured Notes Series J [Member] | LIBOR [Member] |  |
Debt Instrument [Line Items] |  |
Interest rate margin |  | 1.45%
Senior Unsecured Notes Series K [Member] |  |
Debt Instrument [Line Items] |  |
Stated interest rate on debt instrument | 3.44% | 3.44%

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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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- Definition Contractual interest rate for funds borrowed, under the debt agreement. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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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- Definition Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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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- Definition 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. + References No definition available. + Details
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