Earnings Per Common Share (Details) - $ / shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Weighted average number of common shares outstanding - basic (in shares)	73,355	72,754	71,826
Effect of dilutive securities (in shares)	993	720	686
Weighted average number of common shares outstanding - diluted (in shares)	74,348	73,474	72,512
Options outstanding to purchase common stock excluded from computation of diluted earnings per common share (in shares)	1	500	900
Weighted average exercise price of options (in dollars per share)	$ 39.38	$ 34.14	$ 29.34