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Stock-Based Compensation Plans (Narrative) (Details) | 12 Months Ended |  |  |
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Dec. 31, 2018 USD ($) plan $ / shares shares | Dec. 31, 2017 USD ($) $ / shares shares | Dec. 31, 2016 USD ($) $ / shares shares | May 31, 2014 shares | Dec. 31, 2013 shares | Dec. 31, 2012 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Unrecognized compensation cost | $ | $ 23,003 |  |  |  |  |
Employee Stock Option Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Options available for grant (in shares) |  |  |  | 24,000,000 |  |
Options granted net of shares from canceled/expired options (in shares) | 12,787,960 |  |  |  |  |
Unrecognized compensation cost | $ | $ 8,445,267 |  |  |  |  |
Weighted-average period for unrecognized compensation cost expected to be recognized | 1 year 10 months 24 days |  |  |  |  |
Non-Employee Director Stock Option Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Option vesting period | 6 months |  |  |  |  |
Shares approved (in shares) |  |  |  |  |  | 1,000,000
Grants in period (in shares) | 1,074,480 |  |  |  |  |
Option expiration period, years | 10 years |  |  |  |  |
Previous Non-Employee Director Stock Option Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Grants in period (in shares) | 483,940 |  |  |  |  |
Restricted Stock Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares approved (in shares) |  |  | 9,000,000 |  |  |
Restricted period, maximum, in years |  |  | 10 years |  |  |
Shares granted with restriction period (in shares) | 2,637,853 |  |  |  |  |
Shares, granted (in shares) | 6,376,394 |  |  |  |  |
Unearned stock-based compensation | $ | $ 34,824,960 |  |  |  |  |
Amortization expense | $ | $ 8,841,985 | $ 5,353,339 | $ 3,885,042 |  |  |
Restricted Stock Plan, Five Years |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares granted with restriction period (in shares) | 279,420 | 228,630 | 246,660 |  |  |
Restricted period for granted shares, in years | 5 years |  |  |  |  |
Restricted Stock Plan, Four Years |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares granted with restriction period (in shares) | 762,749 | 628,015 |  |  |  |
Restricted period for granted shares, in years | 4 years |  |  |  |  |
Employee Stock Purchase Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares approved (in shares) |  |  |  |  | 2,000,000 |
ESPP discount rate | 85.00% |  |  |  |  |
Discount recognized as compensation expense | 15.00% |  |  |  |  |
Employee Stock Purchase Plan, 2013 |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Shares sold to employees during period (in shares) | 177,846 | 175,479 | 177,781 |  |  |
Cumulative shares sold to employees during period (in shares) | 972,843 |  |  |  |  |
Weighted average fair value of shares sold (in dollars per share) | $ / shares | $ 18.38 |  |  |  |  |
Minimum | Employee Stock Option Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Option vesting period | 1 year |  |  |  |  |
Award expiration period | 5 years |  |  |  |  |
Minimum | Restricted Stock Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Market price (in dollars per share) | $ / shares | $ 20.21 | $ 18.97 | $ 14.70 |  |  |
Maximum | Employee Stock Option Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Option vesting period | 5 years |  |  |  |  |
Award expiration period | 10 years |  |  |  |  |
Maximum | Restricted Stock Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Market price (in dollars per share) | $ / shares | $ 23.14 | $ 21.33 | $ 19.69 |  |  |
Stock Compensation Plan |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of plans | plan | 4 |  |  |  |  |
Employee Stock Option |  |  |  |  |  |
Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |  |  |  |  |  |
Number of plans | plan | 2 |  |  |  |  |

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