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Common Stock Options, Restricted Stock and Changes In Capitalization - Narrative (Details) - USD ($) $ in Thousands |  |  |  |  |  | 12 Months Ended | 44 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 17, 2017 | Dec. 21, 2016 | Mar. 15, 2016 | Jun. 12, 2015 | Apr. 21, 2015 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2016 | Apr. 19, 2013 | Apr. 20, 2006 | Apr. 19, 2006
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Granted for purchase of previous plan's common stock (shares) |  |  |  |  |  | 0 | 0 | 0 |  |  |  |
Proceeds from issuance of common stock, net of offering costs and exercise of stock options |  | $ 698,800 | $ 411,700 | $ 479,700 | $ 501,800 | $ (5,364) | $ 1,110,555 | $ 981,568 |  |  |  |
Vitruvian |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total initial price | $ 1,850,000 |  |  |  |  |  |  |  |  |  |  |
Payments to acquire businesses | $ 1,354,093 |  |  |  |  |  |  |  |  |  |  |
Equity interest issued or issuable, number of shares (in shares) | 23,900,000 |  |  |  |  |  |  |  |  |  |  |
Vitruvian | Indemnity Escrow |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Equity interest issued or issuable, number of shares (in shares) | 5,200,000 |  |  |  |  |  |  |  |  |  |  |
Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in public offering (shares) |  | 33,350,000 | 16,905,000 | 11,500,000 | 10,925,000 | 23,852,117 | 50,255,000 | 22,425,000 |  |  |  |
2005 Stock Incentive Plan | Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Available for grant (shares) |  |  |  |  |  |  |  |  |  |  | 3,000,000 | 1,904,606
Granted for purchase of previous plan's common stock (shares) |  |  |  |  |  | 997,269 |  |  |  |  |  |
1999 Stock Option Plan | Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Granted to employees under previous plan (shares) |  |  |  |  |  |  |  |  |  |  | 627,337 |
2013 Restated Stock Incentive Plan | Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Available for grant (shares) |  |  |  |  |  | 1,939,053 |  |  |  | 7,500,000 | 3,000,000 |
Granted for purchase of previous plan's common stock (shares) |  |  |  |  |  |  |  |  | 627,337 |  |  |
Over-Allotment Option | Common Stock |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Issuance of common stock in public offering (shares) |  | 4,350,000 | 2,205,000 |  |  |  |  |  |  |  |  |
Restricted stock | 2005 Stock Incentive Plan |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Granted for purchase of previous plan's common stock (shares) |  |  |  |  |  | 1,143,217 |  |  |  |  |  |

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