Shareholders' Equity (Narrative) (Details)								12 Months Ended			79 Months Ended	122 Months Ended	138 Months Ended
	Nov. 09, 2016 $ / shares	May 24, 2016 USD ($) $ / shares	Oct. 29, 2015 $ / shares	May 19, 2015 $ / shares shares	Nov. 04, 2014 $ / shares	Jul. 16, 2014	May 02, 2014 $ / shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Jun. 02, 2016 USD ($) shares
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares) | shares								736,171	1,576,090	1,488,230			11,790,894
Aggregate price of shares repurchased since inception | $								$ 20,139,954	$ 37,142,974	$ 29,283,037			$ 142,344,788
Average price of shares repurchased since inception (per share) | $ / shares								$ 27.36	$ 23.57	$ 19.68			$ 12.07
Dividends declared (per share) | $ / shares								$ 0.24	$ 0.22	$ 0.18
Stock split, conversion ratio						1.5
Cash dividends paid | $								$ 12,700,000	$ 11,900,000	$ 9,700,000
Regular Semi-Annual Dividend
Schedule of Shareholders' Equity [Line Items]
Dividends declared (per share) | $ / shares	$ 0.13	$ 0.11	$ 0.11	$ 0.11	$ 0.09		$ 0.09
2010 Stock Repurchase Program [Member]
Schedule of Shareholders' Equity [Line Items]
Stock repurchase program amount authorized (in shares) | shares				5,700,000.0										2,000,000.0
Stock repurchase program amount authorized, value | $		$ 25,000,000												$ 25,000,000
Stock repurchased during period (in shares) | shares								165,598	1,037,590	1,016,717	3,834,819
Aggregate price of shares repurchased since inception | $								$ 4,440,658	$ 24,999,963	$ 19,998,406	$ 60,948,460
Average price of shares repurchased since inception (per share) | $ / shares								$ 26.82	$ 24.09	$ 19.67	$ 15.89
2005 401(k) Stock Repurchase Program [Member]
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares) | shares								540,501	512,754	417,172			6,082,443
Aggregate price of shares repurchased since inception | $								$ 14,875,850	$ 11,557,598	$ 8,246,172			$ 65,732,720
Average price of shares repurchased since inception (per share) | $ / shares								$ 27.52	$ 22.54	$ 19.77			$ 10.81
2006 Director and Officer Repurchase Program [Member]
Schedule of Shareholders' Equity [Line Items]
Stock repurchased during period (in shares) | shares								30,072	25,746	54,341		1,873,632
Aggregate price of shares repurchased since inception | $								$ 823,446	$ 585,413	$ 1,038,459		$ 15,663,608
Average price of shares repurchased since inception (per share) | $ / shares								$ 27.38	$ 22.74	$ 19.11		$ 8.36