Net income Per Share - Reconciliation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Reconciliation of basic earnings per share ("EPS") to diluted EPS
Net income available to common stockholders	$ 43,686	$ 44,361	$ 105,925	$ 113,876	$ 12,757
Basic weighted-average shares outstanding	44,477	46,192	44,673	46,375
Dilutive effect of potential common stock equivalents — restricted stock units	337	430	322	395
Diluted weighted-average shares outstanding	44,814	46,622	44,995	46,770
Earnings per common share:
Basic	$ 0.98	$ 0.96	$ 2.37	$ 2.46
Diluted	$ 0.97	$ 0.95	$ 2.35	$ 2.43