Basis of Presentation - Reconciliation of EPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Reconciliation of basic earnings per share ("EPS") to diluted EPS
Net income available to common stockholders	$ 44,086	$ 28,214	$ 69,515	$ 51,335	$ 41,282
Basic weighted-average shares outstanding (in shares)	46,323	47,634	46,468	47,634
Dilutive effect of potential common stock equivalents — stock options and restricted stock units (in shares)	354	286	374	288
Diluted weighted-average shares outstanding (in shares)	46,677	47,920	46,842	47,922
Earnings per common share:
Basic (in dollars per share)	$ 0.95	$ 0.59	$ 1.50	$ 1.08
Diluted (in dollars per share)	$ 0.94	$ 0.59	$ 1.48	$ 1.07