Capital Stock and Equity Awards - Commons Shares (Details) - USD ($)	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock, shares outstanding	29,917,821	29,696,682
Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares issued during period	221,139
Common stock, shares outstanding	29,917,821	29,696,682	29,467,647	29,257,566
Options | Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares issued during period	177,835
RSUs
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted stock unit dividend equivalents	$ 500,870	$ 434,000
Dividends payable	$ 197,000		$ 154,000
RSUs | Common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of common shares issued during period	43,304