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Equity (Details Textual) - USD ($) shares in Thousands, $ in Millions | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2017 | Sep. 30, 2016 | Jun. 30, 2015 | Mar. 31, 2011
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock, shares authorized | 8,000 |  |  |  |  |  |
Authorized amount of share repurchases |  |  |  |  |  | $ 5,000.0 |
Repurchase of common stock, at cost, shares |  |  | 16,800 |  |  |  |
Payments for repurchase of common stock | $ 1,365.4 | $ 1,000.0 | $ 5,000.0 |  |  |  |
Series A |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock, shares authorized | 1,750 |  |  |  |  |  |
Series X junior participating |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock, shares authorized | 1,000 |  |  |  |  |  |
Undesignated |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Preferred stock, shares authorized | 5,250 |  |  |  |  |  |
2016 Share Repurchase Program |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Authorized amount of share repurchases |  |  |  | $ 3,000.0 | $ 5,000.0 |  |
Repurchase of common stock, at cost, shares | 3,700 | 3,300 |  |  |  |  |
Payments for repurchase of common stock | $ 1,000.0 |  |  |  |  |  |
2011 Share Repurchase Program |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |
Common stock shares authorized for repurchase |  |  |  |  |  |  | 20,000
Repurchase of common stock, at cost, shares | 1,200 |  |  |  |  |  |
Payments for repurchase of common stock | $ 365.4 |  |  |  |  |  |

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