Stockholders' Equity (Details) - USD ($)				3 Months Ended		6 Months Ended		85 Months Ended	128 Months Ended	144 Months Ended
	May 16, 2017	Nov. 09, 2016	May 24, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2017	Jun. 02, 2016	May 19, 2015
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)						296,885	301,505			12,087,779
Aggregate price of shares repurchased since inception						$ 10,433,710	$ 7,673,623			$ 152,778,500
Average price of shares repurchased since inception (per share)						$ 35.14	$ 25.45			$ 12.64
Cash dividends declared per common share				$ 0.13	$ 0.11	$ 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	10,230	3,843,495
Aggregate price of shares repurchased since inception						$ 283,654	$ 265,402	$ 61,232,115
Average price of shares repurchased since inception (per share)						$ 32.69	$ 25.94	$ 15.93
2005 401(k) Stock Repurchase Program
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)						258,051	274,147			6,340,494
Aggregate price of shares repurchased since inception						$ 9,083,606	$ 6,967,393			$ 74,816,326
Average price of shares repurchased since inception (per share)						$ 35.20	$ 25.41			$ 11.80
2006 Director and Officer Repurchase Program
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares)						30,158	17,128		1,903,790
Aggregate price of shares repurchased since inception						$ 1,066,450	$ 440,828		$ 16,730,059
Average price of shares repurchased since inception (per share)						$ 35.36	$ 25.74		$ 8.79