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Equity (Tables) | 6 Months Ended
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Jun. 30, 2013
Targeted or Tracking Stock, Stock [Line Items] |
Schedule of Common Stock Outstanding Roll Forward [Table Text Block] | The following table summarizes issuances of common stock sold through the ATM program since inception through June 30, 2013:
|  | Number of Shares Settled |  | Gross Proceeds |  | Net Proceeds |  | Weighted-average Sales Price
2013: |  |  |  |  |  |  |  |
First quarter |  | 1,889,105 |  |  | $ | 44,459 |  |  | $ | 43,904 |  |  | $ | 23.53 |
Second quarter |  | 6,530,193 |  |  | 167,034 |  |  | 165,692 |  |  | 25.58 |
Total |  | 8,419,298 |  |  | $ | 211,493 |  |  | $ | 209,596 |  |  | $ | 25.12 |

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