CONSOLIDATED STATEMENTS OF CHANGES IN SHAREHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Common Stock	Retained earnings	Accumulated other comprehensive income (loss)	Treasury stock
Beginning Balances (in shares) at Dec. 31, 2015		68,730,731			(7,089,051)
Beginning Balances at Dec. 31, 2015	$ 809,376	$ 571,155	$ 388,240	$ (30,580)	$ (119,439)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	42,382		42,382
Other comprehensive income (loss)	12,892			12,892
Cash dividends declared :
Common stock at $0.34 per share in 2017 and $0.32 per share in 2016	(19,729)		(19,729)
Stock Issued During Period, Shares, Other					57,575
Adjustments to Additional Paid in Capital, Other	0	(971)			$ 971
Excess tax benefit on share-based compensation	156	156
Exercise of stock options, net of shares purchased (in shares)					45,928
Exercise of stock options, net of shares purchased	597	(177)			$ 774
Restricted stock awards, net of forfeitures (in shares)					214,346
Restricted stock awards, net of forfeitures	(1,347)	(4,872)			$ 3,525
Share-based compensation expense	2,396	$ 2,396
Ending Balances (in shares) at Jun. 30, 2016		68,730,731			(6,771,202)
Ending Balances at Jun. 30, 2016	846,723	$ 567,687	410,893	(17,688)	$ (114,169)
Beginning Balances (in shares) at Dec. 31, 2016		68,730,731			(6,751,179)
Beginning Balances at Dec. 31, 2016	865,224	$ 570,382	437,188	(28,443)	$ (113,903)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	47,150		47,150
Other comprehensive income (loss)	6,221			6,221
Cash dividends declared :
Common stock at $0.34 per share in 2017 and $0.32 per share in 2016	(21,088)		(21,088)
Stock Issued During Period, Shares, Other					1,484
Adjustments to Additional Paid in Capital, Other	0	(25)			$ 25
Exercise of stock options, net of shares purchased (in shares)					39,062
Exercise of stock options, net of shares purchased	198	(462)			$ 660
Restricted stock awards, net of forfeitures (in shares)					120,973
Restricted stock awards, net of forfeitures	(2,554)	(3,559)			$ 1,005
Share-based compensation expense	2,966	$ 2,966
Ending Balances (in shares) at Jun. 30, 2017		68,730,731			(6,589,660)
Ending Balances at Jun. 30, 2017	$ 898,117	$ 569,302	$ 463,250	$ (22,222)	$ (112,213)