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]	Treasury Stock [Member]
Shares, Outstanding		73,020
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	$ 0		$ (157)	$ 157
Common Stock, Dividends, Per Share, Cash Paid	$ 0.74
Balance, value at Dec. 31, 2017	$ 1,949,922	$ 1,634,705	337,442	(22,225)
Net income	128,134		128,134
Other comprehensive income (loss)	(47,941)			(47,941)
Issuance of common stock - stock option and other plans, shares		45
Issuance of common stock - stock option and other plans, value	$ 1,857	$ 1,857
Stock Adjustment Deferred Compensation - Shares	0
Stock Adjustment Value Deferred Compensation	$ 7
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		257
Issuance of common stock - restricted stock awards, net of cancelled awards, value	6,231	$ 6,231
Stock Repurchased During Period, Shares		(62)
Stock Repurchased and Retired During Period, Value	(2,660)	$ (2,660)
Cash dividends paid on common stock	(54,155)		(54,155)
Balance, value at Sep. 30, 2018	$ 1,981,395	$ 1,640,140	411,264	(70,009)	$ 0
Shares, Outstanding		73,245
Common Stock, Dividends, Per Share, Cash Paid	$ 0.26
Balance, value at Jun. 30, 2018	$ 1,964,881	$ 1,636,903	383,899	(55,921)
Net income	46,415		46,415
Other comprehensive income (loss)	(14,088)			(14,088)
Issuance of common stock - stock option and other plans, shares		26
Issuance of common stock - stock option and other plans, value	$ 1,121	$ 1,121
Stock Adjustment Deferred Compensation - Shares	0
Stock Adjustment Value Deferred Compensation	$ 1
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		(11)
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,148	$ 2,148
Stock Repurchased During Period, Shares		0
Stock Repurchased and Retired During Period, Value	(33)	$ (33)
Cash dividends paid on common stock	(19,050)		(19,050)
Balance, value at Sep. 30, 2018	$ 1,981,395	$ 1,640,140	411,264	(70,009)	0
Shares, Outstanding		73,260
Shares, Outstanding		73,249
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-02 [Member]			782
Common Stock, Dividends, Per Share, Cash Paid	$ 1.12
Balance, value at Dec. 31, 2018	$ 2,033,649	$ 1,642,246	426,708	(35,305)	0
Net income	148,322		148,322
Other comprehensive income (loss)	100,189			100,189
Issuance of common stock - stock option and other plans, shares		58
Issuance of common stock - stock option and other plans, value	2,025	$ 2,025
Stock Adjustment Deferred Compensation - Shares		0
Stock Adjustment Value Deferred Compensation	2	$ 2
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		354
Issuance of common stock - restricted stock awards, net of cancelled awards, value	6,827	$ 6,827
Stock Repurchased During Period, Shares		(73)
Stock Repurchased and Retired During Period, Value	(2,765)	$ (2,765)
Cash dividends paid on common stock	$ (82,074)		(82,074)
Treasury Stock, Shares, Acquired	(1,300)	(1,300)
Treasury Stock, Value, Acquired, Cost Method	$ (45,380)				(45,380)
Balance, value at Sep. 30, 2019	$ 2,161,577	$ 1,648,335	493,738	64,884	(45,380)
Shares, Outstanding		72,924
Common Stock, Dividends, Per Share, Cash Paid	$ 0.28
Balance, value at Jun. 30, 2019	$ 2,133,638	$ 1,644,922	463,429	47,150	(21,863)
Net income	50,727		50,727
Other comprehensive income (loss)	17,734			17,734
Issuance of common stock - stock option and other plans, shares		31
Issuance of common stock - stock option and other plans, value	1,132	$ 1,132
Stock Adjustment Deferred Compensation - Shares		0
Stock Adjustment Value Deferred Compensation	1	$ 1
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		9
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,299	$ 2,299
Stock Repurchased During Period, Shares		0
Stock Repurchased and Retired During Period, Value	(19)	$ (19)
Cash dividends paid on common stock	$ (20,418)		(20,418)
Treasury Stock, Shares, Acquired	(676)	(676)
Treasury Stock, Value, Acquired, Cost Method	$ (23,517)				23,517
Balance, value at Sep. 30, 2019	$ 2,161,577	$ 1,648,335	$ 493,738	$ 64,884	$ (45,380)
Shares, Outstanding		72,288