Weighted Average Shares Outstanding (Details) - shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Weighted Average Shares Outstanding [Line Items]
Basic (in shares)	54,073,407	54,191,013	55,028,063
Dilutive effect of:
Diluted (in shares)	54,605,298	54,631,313	55,513,219
Stock options [Member]
Dilutive effect of:
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount	46,450	262,336	214,032
Stock options [Member]
Dilutive effect of:
Shares attributable to share-based payment arrangements (in shares)	258,052	166,986	206,778
Performance share awards [Member]
Dilutive effect of:
Shares attributable to share-based payment arrangements (in shares)	273,839	273,314	278,378