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Common Stock Options, Restricted Stock and Changes In Capitalization (Details) - USD ($) |  |  |  | 12 Months Ended | 44 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Feb. 17, 2017 | Dec. 21, 2016 | Mar. 15, 2016 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2016 | May 31, 2018 | Jan. 31, 2018 | 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 |  | $ 698,800,000 | $ 411,700,000 |  | $ (5,364,000) | $ 1,110,555,000 |  |  |  |  |  |
Stock repurchase program, authorized amount |  |  |  |  |  |  |  | $ 200,000,000 | $ 100,000,000 |  |  |
Stock repurchase program, additional authorized amount |  |  |  |  |  |  |  | $ 100,000,000 |  |  |  |
Stock repurchased and retired during period (shares) |  |  |  | 20,700,000 |  |  |  |  |  |  |  |
Stock repurchased and retired during period, net of tax |  |  |  | $ 200,000,000 |  |  |  |  |  |  |  |
Shares repurchased for tax withholding requirement (in shares) |  |  |  | 29,000 |  |  |  |  |  |  |  |
Tax withholding requirement |  |  |  | $ 300,000 |  |  |  |  |  |  |  |
Vitruvian |  |  |  |  |  |  |  |  |  |  |  |
Class of Stock [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Total initial price | $ 1,850,000,000 |  |  |  |  |  |  |  |  |  |  |
Payments to acquire businesses | $ 1,354,093,000 |  |  |  |  |  |  |  |  |  |  |
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 |  |  | 50,255,000 |  |  |  |  |  |
Stock repurchased and retired during period (shares) |  |  |  | 20,746,536 |  |  |  |  |  |  |  |
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) |  |  |  | 3,518,964 |  |  |  |  |  | 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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