Stockholders' Equity (Details) - USD ($)				3 Months Ended		9 Months Ended		88 Months Ended	131 Months Ended	147 Months Ended
	May 16, 2017	Nov. 09, 2016	May 24, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2017	Sep. 30, 2017	Jun. 02, 2016	May 19, 2015
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)						407,360	575,256			12,198,254
Aggregate price of shares repurchased since inception						$ 14,183,646	$ 15,131,237			$ 156,527,435
Average price of shares repurchased since inception (per share)						$ 34.82	$ 26.30			$ 12.83
Cash dividends declared per common share				$ 0	$ 0	$ 0.13	$ 0.11
Dividend Declared
Schedule of Shareholders' Equity [Line Items]
Cash dividends declared per common share	$ 0.13	$ 0.13	$ 0.11
2010 Stock Repurchase Program
Schedule of Shareholders' Equity [Line Items]
Stock repurchase program amount authorized (in shares)											2,000,000.0	5,700,000.0
Stock Repurchase Program, Authorized Amount			$ 25,000,000								$ 25,000,000
Stock repurchased during period (in shares)						8,676	151,266	3,843,495
Aggregate price of shares repurchased since inception						$ 283,654	$ 4,041,685	$ 61,232,115
Average price of shares repurchased since inception (per share)						$ 32.69	$ 26.72	$ 15.93
2005 401(k) Stock Repurchase Program
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)						364,839	402,623			6,447,282
Aggregate price of shares repurchased since inception						$ 12,706,655	$ 10,531,020			$ 78,439,375
Average price of shares repurchased since inception (per share)						$ 34.83	$ 26.16			$ 12.17
2006 Director and Officer Repurchase Program
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)						33,845	21,367		1,907,477
Aggregate price of shares repurchased since inception						$ 1,193,337	$ 558,532		$ 16,855,945
Average price of shares repurchased since inception (per share)						$ 35.26	$ 26.14		$ 8.84