Stockholders' Equity (Stock Rollforward) (Details)	9 Months Ended
	Sep. 30, 2017 shares
Increase (Decrease) in Stock by Class [Roll Forward]
Common stock, beginning of period	18,939,091
Treasury stock, beginning of period	795,758
Changes related to stock-based compensation awards:
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period	(266)
Changes related to deferred compensation plan for Boards of Directors:
Common stock, end of period	18,948,358
Treasury stock, end of period	703,530
Common Shares
Increase (Decrease) in Stock by Class [Roll Forward]
Common stock, beginning of period	18,939,091
Changes related to stock-based compensation awards:
Cancellation of restricted common shares	3,344
Changes related to deferred compensation plan for Boards of Directors:
Common shares issued under dividend reinvestment plan	12,611
Common stock, end of period	18,948,358
Treasury Stock
Increase (Decrease) in Stock by Class [Roll Forward]
Treasury stock, beginning of period	795,758
Changes related to stock-based compensation awards:
Release of restricted common shares	8,719
Cancellation of restricted common shares	4,510
Grant of common shares	(300)
Changes related to deferred compensation plan for Boards of Directors:
Purchase of treasury stock	4,266
Reissuance of treasury stock	(24,634)
Common shares issued under compensation plan for Boards of Directors	(7,404)
Common shares issued under employee stock purchase plan	(8,412)
Treasury stock, end of period	703,530
Restricted Shares | Treasury Stock
Changes related to stock-based compensation awards:
Grant of common shares	(68,707)