Stock-Based Compensation Plans (Narrative) (Details)	12 Months Ended
	Dec. 31, 2017 USD ($) plan $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 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 | $	$ 0
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)	11,404,018
Unrecognized compensation cost | $	$ 9,957,103
Weighted-average period for unrecognized compensation cost expected to be recognized	1 year 9 months
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)	427,000
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)	2,019,404
Shares, granted (in shares)	5,334,225
Unearned stock-based compensation | $	$ 22,417,962
Amortization expense | $	$ 5,353,339	$ 3,885,042	$ 3,486,242
Restricted Stock Plan, Five Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted with restriction period (in shares)	228,630	246,660	229,660
Restricted period for granted shares, in years	5 years
Restricted Stock Plan, Four Years [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares granted with restriction period (in shares)	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)	175,479	177,781	201,785
Cumulative shares sold to employees during period (in shares)	794,997
Weighted average fair value of shares sold (in dollars per share) | $ / shares	$ 17.12
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	$ 18.97	$ 14.70	$ 15.50
Maximum | Employee Stock Option Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option vesting period	5 years
Award expiration period	7 years
Maximum | Restricted Stock Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Market price (in dollars per share) | $ / shares	$ 21.33	$ 19.69	$ 18.30
Stock Compensation Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of plans | plan	4
Employee Stock Option [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of plans | plan	2