Selected Quarterly Financial Data (Unaudited) (Details) (USD $)
In Thousands, except Per Share data, unless otherwise specified
 3 Months Ended 12 Months Ended
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2013
Jun. 30, 2013
Mar. 31, 2013
Dec. 31, 2014
Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]                    
Gross premium written $ 26,361 $ 42,046 $ 22,801 $ 31,224 $ 22,069 $ 32,075 $ 16,562 $ 22,354 $ 122,339 $ 92,487
Net premium earned 27,289 25,575 23,306 21,954 20,512 17,976 16,968 15,888 98,124 71,344
Underwriting income/(loss) 3,164 2,666 1,883 1,422 1,753 1,096 828 298    
Net income attributable to Atlas 9,458 3,493 2,559 2,192 2,178 1,699 1,701 602 17,702 6,180
Net income/(loss) attributable to common shareholders $ 9,434 $ 3,469 $ 2,536 $ 2,169 $ 2,155 $ 3,404 $ 1,476 $ 326 $ 17,608 $ 7,361
Basic earnings/(loss) per common share (USD per share) $ 0.80 $ 0.29 $ 0.24 $ 0.23 $ 0.25 $ 0.41 $ 0.18 $ 0.05 $ 1.61 $ 0.92
Diluted earnings/(loss) per common share (USD per share) $ 0.77 $ 0.29 $ 0.23 $ 0.22 $ 0.22 $ 0.35 $ 0.15 $ 0.05 $ 1.56 $ 0.74