Net Income Per Share - Reconciliation of Basic and Diluted Net Income Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Net Income
Net income available to common stockholders	$ 38,817	$ 36,679
Diluted net income per share	$ 38,817	$ 36,679
Weighted-Average Shares
Net income available to common stockholders (in shares)	49,369,024	49,192,647
Effect of potentially dilutive stock options and restricted stock awards (in shares)	171,820	235,325
Diluted net income per share (in shares)	49,540,844	49,427,972
Per Share Amount
Net income available to common stockholders (in dollars per share)	$ 0.79	$ 0.75
Diluted net income per share (in dollars per share)	$ 0.78	$ 0.74