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Debt - Summary of Unsecured Note Balances, Interest Rate and Maturity Date Information (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Debt Instrument [Line Items] |  |
Net Principal Balance | $ 5,038,812 | $ 4,848,079
Fixed Rate Public Notes [Member] |  |
Debt Instrument [Line Items] |  |
Net Principal Balance | $ 4,591,373 | $ 4,397,829
Weighted average interest rate | 4.61% | 4.90%
Debt instrument maturity year, start | 2020 | 2017
Debt instrument maturity year, end | 2047 | 2045
Fixed Rate Public Notes [Member] | Minimum [Member] |  |
Debt Instrument [Line Items] |  |
Interest Rate Ranges | 2.85% | 2.85%
Fixed Rate Public Notes [Member] | Maximum [Member] |  |
Debt Instrument [Line Items] |  |
Interest Rate Ranges | 7.57% | 7.57%
Floating Rate Public Notes [Member] |  |
Debt Instrument [Line Items] |  |
Net Principal Balance | $ 447,439 | $ 450,250
Weighted average interest rate | 1.82% | 1.28%
Debt instrument, maturity year | 2019 | 2019

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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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- Definition Including the current and noncurrent portions, aggregate carrying amount of all types of notes payable, as of the balance sheet date, with initial maturities beyond one year or beyond the normal operating cycle, if longer. + 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)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
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