Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Earnings Per Share (Details)

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Summary of Significant Accounting Policies - Earnings Per Share (Details) - USD ($)
$ in Thousands
 3 Months Ended 12 Months Ended
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2016
Jun. 30, 2016
Mar. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Accounting Policies [Abstract]                      
Net income attributable to common shareholders $ 23,811 $ 8,493 $ 5,831 $ (644) $ 3,269 $ 5,715 $ 1,994 $ 1,054 $ 37,491 $ 12,032 $ 844
Basic weighted-average shares outstanding (in shares) 45,537,000 45,524,000 45,463,000 45,514,000 45,811,000 46,361,000 46,719,000 46,742,000 45,509,000 46,409,000 46,494,000
Effect of dilutive securities:                      
Stock options (in shares)                 239,000 84,000 1,171,000
Diluted weighted-average shares outstanding (in shares) 45,957,000 45,771,000 45,675,000 45,514,000 45,907,000 46,430,000 46,793,000 46,860,000 45,748,000 46,493,000 47,665,000
Stock options excluded from calculation of dilutive shares as the effect would be anti-dilutive (in shares)                 2,560,353 3,530,220 1,767,778