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Borrowings (Interest Rate Ranges, Maturity and Balance of Indebtedness) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2016 | Dec. 31, 2015
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 4,499,712 | $ 3,427,600
Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | 3,180,624 |
Fixed Rate Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 3,838,300 | 3,148,100
Fixed Rate Debt | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Average Interest Rates |  | 3.92% |
Total outstanding debt |  | $ 1,128,300 | 1,062,900
Fixed Rate Debt | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Average Interest Rates |  | 4.06% |
Total outstanding debt |  | $ 1,860,000 | 1,535,200
Fixed Rate Debt | Interest Rate Swap |  |  |
Debt Instrument [Line Items] |  |  |
Average Interest Rates |  | 2.80% |
Total outstanding debt |  | $ 850,000 | 550,000
Fixed Rate Debt | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 1,128,284 |
Fixed Rate Debt | Minimum | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | 0.0300 |
Debt Instrument Maturity Year |  | 2017 |
Fixed Rate Debt | Minimum | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | .0315 |
Debt Instrument Maturity Year |  | 2017 |
Fixed Rate Debt | Minimum | Interest Rate Swap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | .0263 |
Debt Instrument Maturity Year |  | 2017 |
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 |  | 2025 |
Fixed Rate Debt | Maximum | Interest Rate Swap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate |  | 0.0663 |
Debt Instrument Maturity Year |  | 2018 |
Variable Rate [Domain] | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate, stated percentage |  | 1.65% |
Variable Rate [Domain] | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate, stated percentage |  | 1.08% |
Variable Rate [Domain] | Interest Rate Cap |  |  |
Debt Instrument [Line Items] |  |  |
Interest rate, stated percentage |  | 1.08% |
Variable Rate Debt |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 650,000 | 265,000
Variable Rate Debt | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Average Interest Rates | [1] | 1.08% |
Total outstanding debt | [1] | $ 110,000 | 65,000
Variable Rate Debt | Unsecured Debt |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity Year | [1] | 2020 |
Average Interest Rates | [1] | 1.65% |
Total outstanding debt | [1] | $ 490,000 | 75,000
Interest rate, stated percentage | [1] | 1.65% |
Variable Rate Debt | Secured Debt [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt | [2] | $ 160,000 |
Variable Rate Debt | Interest Rate Cap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate | [1] | 0.0180 |
Average Interest Rates | [1] | 1.08% |
Total outstanding debt | [1] | $ 50,000 | 125,000
Variable Rate Debt | Minimum | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate | [1] | .0180 |
Debt Instrument Maturity Year | [1] | 2017 |
Variable Rate Debt | Minimum | Interest Rate Cap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity Year | [1] | 2017 |
Variable Rate Debt | Maximum | Taxable Secured |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument, Interest Rate, Basis for Effective Rate | [1] | .018 |
Debt Instrument Maturity Year | [1] | 2017 |
Variable Rate Debt | Maximum | Interest Rate Cap |  |  |
Debt Instrument [Line Items] |  |  |
Debt Instrument Maturity Year | [1] | 2017 |
Fair Market Value Adjustment and Debt Issuance Cost [Member] |  |  |
Debt Instrument [Line Items] |  |  |
Total outstanding debt |  | $ 11,400 | $ 14,500
[1] | Amounts are adjusted to reflect interest rate swap and cap agreements in effect at December 31, 2016, and 2015, respectively, which results in us 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.
[2] | Includes capped balances

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