Consolidated Statements of Changes in Shareholders' Equity - USD ($) shares in Thousands, $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Preferred Stock [Member]
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			9
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
Issuance of common stock - restricted stock awards, net of cancelled awards, value	4,114	$ 4,114
Conversion of Stock, Shares Converted					(9)
Conversion of Stock, Amount Converted					$ (2,217)
Conversion of Stock, New Issuance		102
Conversion of Stock, Amount Issued	0	$ 2,217
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		235
Stock Repurchased and Retired During Period, Value	(2,215)	$ (2,215)
Stock Repurchased During Period, Shares		(54)
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			0
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	0		(157)	157
Balance, value at Dec. 31, 2017	1,949,922	$ 1,634,705	337,442	(22,225)	$ 0
Balance, shares at Dec. 31, 2017		73,020			0
Net income	81,719		81,719
Other comprehensive income	(33,853)			(33,853)
Issuance of common stock - stock option and other plans, value	736	$ 736
Issuance of common stock - stock option and other plans, shares		19
Issuance of common stock - restricted stock awards, net of cancelled awards, value	4,083	$ 4,083
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		268
Stock Repurchased and Retired During Period, Value	(2,627)	$ (2,627)
Stock Repurchased During Period, Shares		(62)
Cash dividends paid on common stock	(35,105)		(35,105)
Balance, value at Jun. 30, 2018	1,964,881	$ 1,636,903	$ 383,899	$ (55,921)	$ 0
Balance, shares at Jun. 30, 2018		73,245			0
Stock Adjustment Deferred Compensation - Shares		0
Stock Adjustment Value Deferred Compensation	$ 6	$ 6