Basis of Presentation - Reconciliation of BEPS (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Reconciliation of basic earnings per share ("EPS") to diluted EPS
Net income available to common stockholders	$ 25,429	$ 23,121	$ 69,496
Basic weighted-average shares outstanding (in shares)	46,615	47,616
Dilutive effect of potential common stock equivalents — stock options and restricted stock units (in shares)	394	290
Diluted weighted-average shares outstanding (in shares)	47,009	47,906
Earnings per common share:
Basic (in dollars per share)	$ 0.55	$ 0.49
Diluted (in dollars per share)	$ 0.54	$ 0.48