Earnings Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted-average shares:
Common shares outstanding	78,219,458	78,161,494	80,489,391
Restricted common shares	556,592	653,944	615,611
Total weighted average shares	78,776,050	78,815,438	81,105,002
Percentage of weighted-average shares:
Common shares outstanding	99.29%	99.17%	99.24%
Restricted common shares	0.71%	0.83%	0.76%
Total	100.00%	100.00%	100.00%