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Consolidated Statement of Shareholders' Equity Consolidated Statement of Shareholders' Equity (parenthetical) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]      
Public offering costs $ 4.9 $ 8.5 $ 8.9