Stockholders' Equity (Details) - USD ($)				1 Months Ended	3 Months Ended		9 Months Ended		100 Months Ended	143 Months Ended	159 Months Ended
	May 18, 2018	Nov. 07, 2017	May 16, 2017	Oct. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2018	Sep. 30, 2018	May 15, 2018	Jun. 02, 2016	May 24, 2016	May 19, 2015
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)							513,875	407,360			12,826,903
Aggregate price of shares repurchased since inception							$ 18,360,416	$ 14,183,646			$ 178,939,369
Average price of shares repurchased since inception (per share)							$ 35.73	$ 34.82			$ 13.95
Cash dividends declared per common share:					$ 0	$ 0	$ 0.16	$ 0.13
Dividend Declared
Schedule of Shareholders' Equity [Line Items]
Cash dividends declared per common share:	$ 0.16	$ 0.13	$ 0.13
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												$ 15,000,000.0	$ 25,000,000.0	$ 25,000,000.0
Stock repurchased during period (in shares)							116,289	8,676	3,959,784
Aggregate price of shares repurchased since inception							$ 3,839,766	$ 283,654	$ 65,071,881
Average price of shares repurchased since inception (per share)							$ 33.02	$ 32.69	$ 16.43
2010 Stock Repurchase Program | Subsequent Event
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)				85,200
Aggregate price of shares repurchased since inception				$ 2,800,000
2005 401(k) Stock Repurchase Program
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)							373,129	364,839			6,923,152
Aggregate price of shares repurchased since inception							$ 13,660,954	$ 12,706,655			$ 95,729,759
Average price of shares repurchased since inception (per share)							$ 36.61	$ 34.83			$ 13.83
2005 401(k) Stock Repurchase Program | Subsequent Event
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)				30,618
Aggregate price of shares repurchased since inception				$ 1,000,000
2006 Director and Officer Repurchase Program
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)							24,457	33,845		1,943,967
Aggregate price of shares repurchased since inception							$ 859,696	$ 1,193,337		$ 18,137,729
Average price of shares repurchased since inception (per share)							$ 35.15	$ 35.26		$ 9.33