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Borrowings (Indebtedness) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017
Debt Instrument [Line Items] |  |
Total outstanding debt | $ 4,530,000 | $ 4,502,057
Average interest rate (percentage) | 3.80% |
Secured Debt [Member] |  |
Debt Instrument [Line Items] |  |
Average interest rate (percentage) | 4.60% |
Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Total outstanding debt | $ 4,053,302 | 3,525,765
Average interest rate (percentage) | 3.70% |
Mortgages |  |
Debt Instrument [Line Items] |  |
Average interest rate (percentage) | 4.44% |
$1 billion unsecured revolving credit faciltiy [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Average interest rate (percentage) | 3.42% |
Wells Fargo Term Loan [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Debt Instrument, Face Amount | $ 300,000 | 550,000
Average interest rate (percentage) | 2.30% |
Contract maturity | Mar. 01, 2022 |
Variable Rate Debt [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Average interest rate (percentage) | 3.30% |
Contract maturity | Jan. 22, 2020 |
Unsecured Debt | $ 600,000 | 300,000
$1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Line of credit facility, amount outstanding | 540,000 |
Secured Debt [Member] | Secured Debt [Member] |  |
Debt Instrument [Line Items] |  |
Total outstanding debt | 475,026 | 976,292
Variable Rate Credit Facility [Member] | FNMA | Secured Debt [Member] |  |
Debt Instrument [Line Items] |  |
Line of credit facility, amount outstanding | $ 0 | 80,000
Contract maturity |  |
Fixed Rate Debt | Mortgages | Secured Debt [Member] |  |
Debt Instrument [Line Items] |  |
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate | $ 476,161 | 882,752
Average interest rate (percentage) | 4.60% |
Contract maturity | Feb. 23, 2031 |
Fixed Rate Debt | Senior Notes | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Debt Instrument, Face Amount | $ 2,642,000 | 2,292,000
Average interest rate (percentage) | 4.00% |
Contract maturity | Jul. 12, 2025 |
Variable Rate Debt [Member] | $1 billion unsecured revolving credit faciltiy [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Line of credit facility, amount outstanding | $ 540,000 | 410,000
Average interest rate (percentage) | 3.40% |
Contract maturity | Apr. 15, 2020 |
Fair Market Value Adjustment and Debt Issuance Cost [Member] | Secured Debt [Member] |  |
Debt Instrument [Line Items] |  |
Line of credit facility, amount outstanding | $ (1,135) | 13,540
Fair Market Value Adjustment and Debt Issuance Cost [Member] | $1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Debt Instrument, Face Amount | $ (28,698) | $ (26,235)
Minimum | $1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Debt Instrument, Basis Spread on Variable Rate | 0.85% |
Maximum | $1 billion unsecured revolving credit facility [Member] | Unsecured Debt |  |
Debt Instrument [Line Items] |  |
Debt Instrument, Basis Spread on Variable Rate | 1.55% |

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