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Stock-based Compensation Expense Restricted and Performance Stock Unit Activity (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Net of Forfeitures [Abstract]      
Outstanding, Units 458,792 442,993 463,893
Outstanding, Weighted Average Grant Date Fair Value $ 34.68 $ 32.84 $ 28.96
Granted, Units 202,145 193,734 181,797
Granted, Weighted Average Grant Date Fair Value $ 41.96 $ 35.89 $ 34.77
Forfeited, Units (64,840) (3,044) (14,988)
Forfeited, Weighted Average Grant Date Fair Value $ 39.57 $ 28.62 $ 34.10
Vested, Units (196,721) (174,891) (187,709)
Vested, Weighted Average Grant Date Fair Value $ 31.78 $ 31.47 $ 25.82
Outstanding, Units 399,376 458,792 442,993
Outstanding, Weighted Average Grant Date Fair Value $ 37.98 $ 34.68 $ 32.84