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Earnings per Common Share Basic Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2018
Jun. 30, 2017
Jun. 30, 2018
Jun. 30, 2017
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Net income $ 29,827 $ 19,266 $ 99,858 $ 60,020
Weighted-average Class A shares issued and outstanding 52,105 50,013 51,774 50,234
Basic earnings per Class A common share, in usd per share $ 0.57 $ 0.39 $ 1.93 $ 1.19
Diluted weighted-average Class A shares issued and outstanding 54,390 52,452 54,301 52,577
Diluted earnings per Class A common share, in usd per share $ 0.55 $ 0.37 $ 1.84 $ 1.14
Stock Options [Member]        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Dilutive potential common shares 343 759 441 706
Restricted Stock Units (RSUs) [Member]        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Dilutive potential common shares 1,236 1,418 1,296 1,368
Performance Shares [Member]        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Dilutive potential common shares 705 259 789 268
Employee Stock Purchase Plan [Member]        
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]        
Dilutive potential common shares 1 3 1 1