Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Preferred Stock [Member]
Balance, value at Dec. 31, 2015	$ 1,242,128	$ 990,281	$ 255,925	$ (6,295)	$ 2,217
Balance, shares at Dec. 31, 2015		57,724,000			9,000
Net income	46,664		46,664
Other comprehensive income	27,106			27,106
Issuance of common stock - stock option and other plans, value	603	$ 603
Issuance of common stock - stock option and other plans, shares		21,000
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,551	$ 2,551
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		317,000
Stock Repurchased and Retired During Period, Value	(1,092)	$ (1,092)
Stock Repurchased During Period, Shares		(37,000)
Dividends, Preferred Stock, Cash	(77)		(77)
Cash dividends paid on common stock	(43,404)		(43,404)
Balance, value at Jun. 30, 2016	1,274,479	$ 992,343	259,108	20,811	$ 2,217
Balance, shares at Jun. 30, 2016		58,025,000			9,000
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-09 [Member]	67	$ 184	(117)
Balance, value at Dec. 31, 2016	1,251,012	$ 995,837	271,957	(18,999)	$ 2,217
Balance, shares at Dec. 31, 2016		58,042,000			8,782
Net income	56,331		56,331
Other comprehensive income	12,471			12,471
Issuance of common stock - stock option and other plans, value	1,155	$ 1,155
Issuance of common stock - stock option and other plans, shares		28,000
Issuance of common stock - restricted stock awards, net of cancelled awards, value	4,114	$ 4,114
Conversion of Stock, Shares Converted					(9,000)
Conversion of Stock, Amount Converted					$ (2,217)
Conversion of Stock, New Issuance		102,000
Conversion of Stock, Amount Issued	0	$ 2,217
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		235,000
Stock Repurchased and Retired During Period, Value	(2,215)	$ (2,215)
Stock Repurchased During Period, Shares		(54,000)
Cash dividends paid on common stock	(25,621)		(25,621)
Balance, value at Jun. 30, 2017	$ 1,297,314	$ 1,001,292	$ 302,550	$ (6,528)	$ 0
Balance, shares at Jun. 30, 2017		58,353,000			0