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	29,199		29,199
Other comprehensive income	6,393			6,393
Issuance of common stock - stock option and other plans, value	1,145	$ 1,145
Issuance of common stock - stock option and other plans, shares		28
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,358	$ 2,358
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		207
Stock Repurchased and Retired During Period, Value	(2,039)	$ (2,039)
Stock Repurchased During Period, Shares		(50)
Cash dividends paid on common stock	(12,792)		(12,792)
Balance, value at Mar. 31, 2017	1,275,343	$ 999,702	288,247	(12,606)	$ 0
Balance, shares at Mar. 31, 2017		58,329			0
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	(46)		(203)	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	39,970		39,970
Other comprehensive income	(26,065)			(26,065)
Issuance of common stock - stock option and other plans, value	719	$ 719
Issuance of common stock - stock option and other plans, shares		17
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,064	$ 2,064
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		263
Stock Repurchased and Retired During Period, Value	(2,575)	$ (2,575)
Stock Repurchased During Period, Shares		(60)
Cash dividends paid on common stock	(16,069)		(16,069)
Balance, value at Mar. 31, 2018	1,947,923	$ 1,634,916	$ 361,140	$ (48,133)	$ 0
Balance, shares at Mar. 31, 2018		73,240			0
Stock Adjustment Deferred Compensation - Shares		0
Stock Adjustment Value Deferred Compensation	$ 3	$ 3