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Fair Value Measurements - Narrative (Details) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Retail | Minimum |  |
Debt Instrument [Line Items] |  |
Fair value assumption, interest rate | 5.03% | 4.74%
Retail | Maximum |  |
Debt Instrument [Line Items] |  |
Fair value assumption, interest rate | 6.97% | 7.13%
7.25% 2063 Senior Notes |  |
Debt Instrument [Line Items] |  |
Interest rate on debt | 7.25% |
7.25% 2064 Senior Notes |  |
Debt Instrument [Line Items] |  |
Interest rate on debt | 7.25% |
6.95% Senior Notes |  |
Debt Instrument [Line Items] |  |
Interest rate on debt | 6.95% |
6.7% Senior Notes |  |
Debt Instrument [Line Items] |  |
Interest rate on debt | 6.70% |

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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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