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Shareholder's Equity (Details) - USD ($) | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Stockholders Equity Note [Line Items] |  |  |
Common stock, shares outstanding | 113,643,166 | 113,518,212 |
Common shares and operating partnership units outstanding | 117,834,752 | 117,738,615 |
Treasury Stock, Shares, Acquired | 47,956 | 22,067 |
Preferred Stock, Liquidation Preference, Value | $ 43,400,000 |  |
Preferred Stock, Liquidation Preference Per Share | $ 50.00 |  |
Preferred Stock, Redemption Date | Oct. 01, 2026 |  |
Preferred Stock, Redemption Price Per Share | $ 50.00 |  |
Preferred Stock, Dividend Rate, Percentage | 8.50% |  |
Preferred Stock, Dividend Rate, Per-Dollar-Amount | $ 4.25 |  |
Dividend And Distribution Reinvestment And Share Purchase Plan |  |  |
Stockholders Equity Note [Line Items] |  |  |
Common stock issued for shareholders to reinvest distributions (shares) | 9,568 | 7,906 | 8,562
Dividend And Distribution Reinvestment And Share Purchase Plan | Minimum |  |  |
Stockholders Equity Note [Line Items] |  |  |
Optional cash investments in common shares | $ 250 |  |
Dividend And Distribution Reinvestment And Share Purchase Plan | Maximum |  |  |
Stockholders Equity Note [Line Items] |  |  |
Optional cash investments in common shares | $ 5,000 |  |
Common stock registered for Dividend and Distribution Reinvestment and Share Purchase Plan (shares) | 9,600,000 |  |
Discount rate on sale of common stock (percent) | 5.00% |  |
Parent Company [Member] |  |  |
Stockholders Equity Note [Line Items] |  |  |
Preferred Stock, Shares Outstanding | 867,846 | 867,846 |
Common stock, shares outstanding | 113,643,166 | 113,518,212 |
Percentage of Operating Partnership net income (percent) | 96.40% | 95.00% | 94.80%
Noncontrolling Interest in Joint Ventures | $ 2,306,000 | $ 2,306,000 |
Common stock registered for Dividend and Distribution Reinvestment and Share Purchase Plan (shares) | 145,000,000 | 145,000,000 |
Proceeds from Issuance of Common Stock | $ 1,557,000 | $ 291,000 | $ 622,000
Proceeds from Stock Options Exercised | $ 432,000 | $ 0 | $ 420,000
Preferred Stock, Liquidation Preference Per Share | $ 50 | $ 50 |
Employee Stock Option [Member] |  |  |
Stockholders Equity Note [Line Items] |  |  |
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Number | 108,438 | 147,282 |
Proceeds from Stock Options Exercised | $ 400,000 | $ 0 |
Noncontrolling Interest |  |  |
Stockholders Equity Note [Line Items] |  |  |
Common Shares Issuable Upon Conversion Of Convertible Stock | 4,191,586 | 4,220,403 |
Limited Partners' Capital Account | $ 421,500,000 | $ 413,300,000 |
Redeemable Capital Shares Par Or Stated Value Per Share | $ 100.56 | $ 97.92 |
Percentage of Operating Partnership net income (percent) | 3.60% | 5.00% | 5.20%

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