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]
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	$ (46)		$ (203)	$ 157
Balance, shares at Dec. 31, 2017		73,020
Balance, value at Dec. 31, 2017	1,949,922	$ 1,634,705	337,442	(22,225)
Net income	39,970		39,970
Other comprehensive income	(26,065)			(26,065)
Issuance of common stock - stock option and other plans, shares		17
Issuance of common stock - stock option and other plans, value	$ 719	$ 719
Stock Adjustment Deferred Compensation - Shares	0
Stock Adjustment Value Deferred Compensation	$ 3
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		263
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,064	$ 2,064
Stock Repurchased During Period, Shares		(60)
Stock Repurchased and Retired During Period, Value	(2,575)	$ (2,575)
Cash dividends paid on common stock	(16,069)		(16,069)
Balance, shares at Mar. 31, 2018		73,240
Balance, value at Mar. 31, 2018	1,947,923	$ 1,634,916	361,140	(48,133)
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-02 [Member]			782
Balance, shares at Dec. 31, 2018		73,249
Balance, value at Dec. 31, 2018	2,033,649	$ 1,642,246	426,708	(35,305)
Net income	45,871		45,871
Other comprehensive income	38,351			38,351
Issuance of common stock - stock option and other plans, shares		25
Issuance of common stock - stock option and other plans, value	878	$ 878
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		355
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,285	$ 2,285
Stock Repurchased During Period, Shares		(64)
Stock Repurchased and Retired During Period, Value	(2,432)	$ (2,432)
Cash dividends paid on common stock	(30,764)		(30,764)
Balance, shares at Mar. 31, 2019		73,565
Balance, value at Mar. 31, 2019	$ 2,088,620	$ 1,642,977	$ 442,597	$ 3,046