Basis of Presentation - Reconciliation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Reconciliation of basic earnings per share ("EPS") to diluted EPS
Net income available to common stockholders	$ 44,361	$ 28,197	$ 113,876	$ 79,532	$ 13,085
Basic weighted-average shares outstanding (in shares)	46,192	47,367	46,375	47,544
Dilutive effect of potential common stock equivalents — stock options and restricted stock units (in shares)	430	319	395	299
Diluted weighted-average shares outstanding (in shares)	46,622	47,686	46,770	47,843
Earnings per common share:
Basic (in dollars per share)	$ 0.96	$ 0.60	$ 2.46	$ 1.67
Diluted (in dollars per share)	$ 0.95	$ 0.59	$ 2.43	$ 1.66