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Borrowings (Interest Rate Ranges, Maturity and Balance of Indebtedness) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 3.60% |
Total outstanding debt |  | $ 4,500,000 | $ 4,499,700
Debt Instrument, Maturity Date |  | Mar. 11, 2022 |
Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 3.50% |
Total outstanding debt |  | $ 3,525,765 |
Debt Instrument, Maturity Date |  | Dec. 19, 2022 |
Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 3.80% |
Debt Instrument, Maturity Date |  | Sep. 13, 2019 |
Variable Rate Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Effective Percentage |  | 2.30% |
Unsecured Debt |  | $ 300,000 |
Variable Rate Debt [Member] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Maturity Date |  | Aug. 29, 2020 |
Secured Debt [Member] | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 976,292 |
Fixed Rate Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 3,724,800 | 3,838,300
Fixed Rate Debt | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Average Interest Rates |  | 3.97% |
Total outstanding debt |  | $ 882,800 | 1,128,300
Fixed Rate Debt | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Average Interest Rates |  | 3.97% |
Total outstanding debt |  | $ 2,292,000 | 1,860,000
Fixed Rate Debt | Interest Rate Swap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity Year |  | 2018 |
Average Interest Rates |  | 2.96% |
Total outstanding debt |  | $ 550,000 | 850,000
Fixed Rate Debt | Minimum | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | 0.0300 |
Debt Instrument Maturity Year |  | 2018 |
Fixed Rate Debt | Minimum | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | .0338 |
Debt Instrument Maturity Year |  | 2018 |
Fixed Rate Debt | Minimum | Interest Rate Swap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | .0245 |
Fixed Rate Debt | Maximum | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | 0.0549 |
Debt Instrument Maturity Year |  | 2025 |
Fixed Rate Debt | Maximum | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | 0.0557 |
Debt Instrument Maturity Year |  | 2027 |
Fixed Rate Debt | Maximum | Interest Rate Swap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | 0.0355 |
Variable Rate [Domain] | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate, stated percentage |  | 1.76% |
Variable Rate [Domain] | Interest Rate Cap |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate, stated percentage |  | 1.76% |
Variable Rate Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 790,000 | 650,000
Variable Rate Debt | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity Year | [1] | 2018 |
Average Interest Rates | [1] | 1.76% |
Total outstanding debt | [1] | $ 55,000 | 110,000
Variable Rate Debt | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Average Interest Rates | [1] | 2.41% |
Total outstanding debt | [1] | $ 710,000 | 490,000
Variable Rate Debt | Interest Rate Cap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate | [1] | 0.0176 |
Debt Instrument Maturity Year | [1] | 2018 |
Average Interest Rates | [1] | 1.76% |
Total outstanding debt | [1] | $ 25,000 | 50,000
Variable Rate Debt | Minimum | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate | [1] | .0176 |
Variable Rate Debt | Minimum | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity Year | [1] | 2020 |
Interest rate, stated percentage | [1] | 2.31% |
Variable Rate Debt | Maximum | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate | [1] | .0176 |
Variable Rate Debt | Maximum | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity Year | [1] | 2021 |
Interest rate, stated percentage | [1] | 2.47% |
Fair Market Value Adjustment and Debt Issuance Cost [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ (12,700) | $ 11,400
[1] | Amounts are adjusted to reflect interest rate swap and cap agreements in effect at December 31, 2017, and 2016, respectively, which results in paying fixed interest payments over the terms of the interest rate swaps and on changes in interest rates above the strike rate of the cap.

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