Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended	204 Months Ended
	Nov. 16, 2016	Nov. 30, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Stockholders' Equity Note [Abstract]
Common Stock Shares Authorized			100,000,000	100,000,000	100,000,000
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserved Shares Dividend Reinvestment and Direct Stock Purchase Plan			224,438	224,438
Total shares issued in offering	1,012,000	880,000
Offering price (in usd per share)	$ 54.63
Net proceeds from shares issued in offering	$ 52,800		$ 52,761
Underwriters [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Option period to purchase additional shares		30 days
Total shares issued in offering		132,000
Stock Repurchase Plan [Member]
Equity, Class of Treasury Stock [Line Items]
Stock Repurchase Program Expiration Date			May 31, 2017
Stock Repurchase Program, Number of Shares Authorized to be Repurchased			2,800,000	2,800,000
Stock Repurchase Program, Authorized Amount			$ 100,000	$ 100,000
Treasury Stock, Shares, Acquired			0	2,100,000
Treasury Stock Program				$ 83,300
Employee Stock Purchase Plan Member [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserved Shares Employee Stock Purchase Plan			60,661	60,661
Restated Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding			180,163	180,163	352,688	416,088	492,150