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Summary of Significant Accounting Policies (Details 2) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Change in Accounting Estimate [Line Items] |  |  |
Approximate amount by which basis of assets and liabilities for tax reporting purposes is higher than amount reported on consolidated balance sheet | $ 150,000 |  |
Deferred lease related compensation costs | 1,100 | $ 1,100 | $ 1,200
Future minimum payments due | $ 90,832 |  |
Common Shares [Member] |  |  |
Change in Accounting Estimate [Line Items] |  |  |
Ordinary income (as a percent) | 86.50% | 48.00% | 38.30%
Long-term capital gain (as a percent) | 0.00% | 0.00% | 61.70%
Return of capital (as a percent) | 13.50% | 52.00% | 0.00%
Preferred Shares [Member] |  |  |
Change in Accounting Estimate [Line Items] |  |  |
Ordinary income (as a percent) | 100.00% | 100.00% | 38.30%
Long-term capital gain (as a percent) | 0.00% | 0.00% | 61.70%
Return of capital (as a percent) | 0.00% | 0.00% | 0.00%
Ground Leases [Member] |  |  |
Change in Accounting Estimate [Line Items] |  |  |
Future minimum payments due | $ 89,900 |  |
Accounting Standards Update 2017-05 [Member] |  |  |
Change in Accounting Estimate [Line Items] |  |  |
Effect of change on net income |  | $ 18,000 |

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