Net income Per Share - Reconciliation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Reconciliation of basic earnings per share ("EPS") to diluted EPS
Net income available to common stockholders	$ 39,577	$ 44,086	$ 62,239	$ 69,515	$ 57,118
Basic weighted-average shares outstanding	44,671	46,323	44,772	46,468
Dilutive effect of potential common stock equivalents — restricted stock units	301	354	317	374
Diluted weighted-average shares outstanding	44,972	46,677	45,089	46,842
Earnings per common share:
Basic	$ 0.89	$ 0.95	$ 1.39	$ 1.50
Diluted	$ 0.88	$ 0.94	$ 1.38	$ 1.48