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Forward Equity and Equity Distribution Program (Details) - USD ($) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Class of Stock |  |  |  |
Proceeds from issuance of common stock | $ 41,900,000 | $ 465,800,000 | $ 57,800,000 |
Common stock, shares outstanding (in shares) | 58,500,000 | 58,500,000 | 55,800,000 | 55,800,000
Equity Distribution Program |  |  |  |
Class of Stock |  |  |  |
Stock program, maximum amount authorized | $ 500,000,000 |  |  |
Proceeds from issuance of common stock | $ 0 | $ 440,300,000 |  |
Maximum number of shares available for sale under agreement (in shares) |  | 2,900,000 |  |
Common stock price (in dollars per share) |  | $ 167.25 |  |
Number of common shares issued (in shares) |  | 2,700,000 |  |
Number of shares net settled for cash (in shares) |  | 200,000 |  |
Average share price of shares net settled for cash (in dollars per share) |  | $ 144.59 |  |
Common stock, shares outstanding (in shares) |  | 0 |  |

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