Significant Accounting Policies (Details) - shares shares in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Weighted Average Number of Shares Outstanding, Basic	44,147	44,141	44,164	44,159	44,147	44,110	43,297	42,380	44,153	43,479	42,747
Incremental Common Shares Attributable to Contingently Issuable Shares									69	127	98
Weighted Average Number of Shares, Restricted Stock									42	64	63
Weighted Average Number of Shares Outstanding, Diluted	44,276	44,332	44,374	44,347	44,326	44,320	43,505	42,520	44,264	43,670	42,908