Consolidated Statement of Changes in Stockholders' Equity - USD ($) $ in Thousands	Total	Common stock	Retained earnings	Accumulated other comprehensive income (loss)
Beginning Balance at Dec. 31, 2015	$ 950,493	$ 311,720	$ 638,367	$ 406
Comprehensive income:
Net income	70,871		70,871
Other comprehensive income, net of tax expense	8,416			8,416
Common stock transactions:
Common shares purchased and retired	(26,119)	(26,119)
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	3,657	3,657
Stock-based compensation expense	3,350	3,350
Tax benefit of stock-based compensation	1,352	1,352
Cash dividends declared:
Common cash dividend declared	(29,516)		(29,516)
Ending Balance at Sep. 30, 2016	982,504	293,960	679,722	8,822
Beginning Balance at Dec. 31, 2016	982,593	296,071	694,650	(8,128)
Comprehensive income:
Net income	72,253		72,253
Other comprehensive income, net of tax expense	9,151			9,151
Common stock transactions:
Common shares purchased and retired	(938)	(938)
Common shares issued	385,969	385,969
Stock options exercised, net of shares tendered in payment of option price and income tax withholding amounts	1,988	1,988
Stock-based compensation expense	3,418	3,418
Cash dividends declared:
Common cash dividend declared	(35,024)		(35,024)
Ending Balance at Sep. 30, 2017	$ 1,419,410	$ 686,508	$ 731,879	$ 1,023