Annual report pursuant to Section 13 and 15(d)

Consolidated Statements of Stockholders' Equity (Parenthetical)

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Consolidated Statements of Stockholders' Equity (Parenthetical) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Increase (Decrease) in Stockholders' Equity [Roll Forward]      
Dividends declared per share (in dollars per share) $ 0.270 $ 0.235 $ 0.215