Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended		216 Months Ended
	Nov. 16, 2016	Nov. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2015	Dec. 31, 2014
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			155,086		155,086
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
Stock Repurchase Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock Repurchase Program Expiration Date			May 31, 2018
Stock Repurchase Program, Authorized Amount			$ 100,000		$ 100,000
Stock Repurchase Program, Number of Shares Authorized to be Repurchased			2,800,000		2,800,000
Treasury Stock, Shares, Acquired			0		2,100,000
Treasury Stock Program					$ 83,300
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
Employee Stock Purchase Plan Member [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Reserved Shares Employee Stock Purchase Plan			43,058		43,058
Restated Stock Option Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options outstanding			91,688	180,163	91,688	352,688	416,088