Condensed Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Common Stock	Surplus	Retained Earnings	Accumulated Other Comprehensive Income (Loss).	Total
Balance at Dec. 31, 2014	$ 60,377	$ 701,764	$ 223,156	$ (377)	$ 984,920
Balance (in shares) at Dec. 31, 2014	24,150,702
Increase (Decrease) in Stockholders' Equity
Net income			73,938		73,938
Other Comprehensive Income, net of tax effects				2,252	2,252
Total comprehensive income					76,190
Cash dividends on Series A preferred stock			(17,413)		(17,413)
Stock options exercised	$ 77	881			958
Stock options exercised (in shares)	30,560
Employee stock purchases	$ 17	427			444
Employee stock purchases (in shares)	6,971
Restricted stock awards	$ 102	(102)
Restricted stock awards (in shares)	41,105
Common stock repurchased	$ (44)	(1,057)			(1,101)
Common stock repurchased (in shares)	(17,545)
Share-based compensation expense		4,314			4,314
Balance at Sep. 30, 2015	$ 60,529	706,227	279,681	1,875	$ 1,048,312
Balance (in shares) at Sep. 30, 2015	24,211,793				24,211,793
Balance (in shares) Preferred Stock at Sep. 30, 2015					0
Balance at Dec. 31, 2015	$ 60,407	703,929	298,919	(3,871)	$ 1,059,384
Balance (in shares) at Dec. 31, 2015	24,162,657				24,162,657
Balance (in shares) Preferred Stock at Dec. 31, 2015					0
Increase (Decrease) in Stockholders' Equity
Net income			77,105		$ 77,105
Other Comprehensive Income, net of tax effects				7,866	7,866
Total comprehensive income					84,971
Cash dividends on Series A preferred stock			(21,534)		(21,534)
Stock options exercised	$ 111	1,402			1,513
Stock options exercised (in shares)	44,491
Employee stock purchases	$ 19	455			474
Employee stock purchases (in shares)	7,793
Restricted stock awards	$ 99	(99)
Restricted stock awards (in shares)	39,431
Stock issued pursuant to restricted stock units.	$ 90	(90)
Stock issued pursuant to restricted stock units (in shares)	35,903
Value of common stock repurchased buyback plan	$ (82)	(2,048)			(2,130)
Common stock repurchased buyback plan (in shares)	(32,900)
Common stock repurchased	$ (121)	(3,129)			(3,250)
Common stock repurchased (in shares)	(48,253)
Share-based compensation expense		4,704			4,704
Balance at Sep. 30, 2016	$ 60,523	$ 705,124	$ 354,490	$ 3,995	$ 1,124,132
Balance (in shares) at Sep. 30, 2016	24,209,122				24,209,122
Balance (in shares) Preferred Stock at Sep. 30, 2016					0