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Summary of Significant Accounting Policies Net income (Loss) Per Common Share (Details) - shares
shares in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]      
Weighted average common shares outstanding (basic) 174,721 173,325 175,721
Impact of common stock equivalents 1,211 1,093 1,053
Weighted average common shares outstanding (diluted) 175,932 174,418 176,774
Stock Options      
Weighted Average Number of Shares Outstanding Reconciliation [Abstract]      
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount   500 1,700