Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]
Shares, Outstanding		73,020,000
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)
Net income	81,719		81,719
Other comprehensive income	(33,853)			(33,853)
Issuance of common stock - stock option and other plans, shares		19,000
Issuance of common stock - stock option and other plans, value	$ 736	$ 736
Stock Adjustment Deferred Compensation - Shares	0
Stock Adjustment Value Deferred Compensation	$ 6
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		268,000
Issuance of common stock - restricted stock awards, net of cancelled awards, value	4,083	$ 4,083
Stock Repurchased During Period, Shares		(62,000)
Stock Repurchased and Retired During Period, Value	(2,627)	$ (2,627)
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
Shares, Outstanding		73,240,000
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-01 [Member]	46		46	0
Balance, value at Mar. 31, 2018	1,947,923	$ 1,634,916	361,140	(48,133)
Net income	41,749		41,749
Other comprehensive income	(7,788)			(7,788)
Issuance of common stock - stock option and other plans, shares		2,000
Issuance of common stock - stock option and other plans, value	$ 17	$ 17
Stock Adjustment Deferred Compensation - Shares	0
Stock Adjustment Value Deferred Compensation	$ 3
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		5,000
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,019	$ 2,019
Stock Repurchased During Period, Shares		(2,000)
Stock Repurchased and Retired During Period, Value	(52)	$ (52)
Cash dividends paid on common stock	(19,036)		(19,036)
Balance, value at Jun. 30, 2018	1,964,881	$ 1,636,903	383,899	(55,921)	0
Shares, Outstanding		73,245,000
Shares, Outstanding		73,249,000
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-02 [Member]			782
Balance, value at Dec. 31, 2018	2,033,649	$ 1,642,246	426,708	(35,305)	0
Net income	97,595		97,595
Other comprehensive income	82,455			82,455
Issuance of common stock - stock option and other plans, shares		27,000
Issuance of common stock - stock option and other plans, value	893	$ 893
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		345,000
Issuance of common stock - restricted stock awards, net of cancelled awards, value	4,528	$ 4,528
Stock Repurchased During Period, Shares		(73,000)
Stock Repurchased and Retired During Period, Value	(2,746)	$ (2,746)
Cash dividends paid on common stock	$ (61,656)		(61,656)
Treasury Stock, Shares, Acquired	(624,158)	(624,000)
Treasury Stock, Value, Acquired, Cost Method	$ (21,863)				(21,863)
Balance, value at Jun. 30, 2019	2,133,638	$ 1,644,922	463,429	47,150	(21,863)
Shares, Outstanding		73,565,000
Balance, value at Mar. 31, 2019	2,088,620	$ 1,642,977	442,597	3,046	0
Net income	51,724		51,724
Other comprehensive income	44,104			44,104
Issuance of common stock - stock option and other plans, shares		2,000
Issuance of common stock - stock option and other plans, value	15	$ 15
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		(10,000)
Issuance of common stock - restricted stock awards, net of cancelled awards, value	2,243	$ 2,243
Stock Repurchased During Period, Shares		(9,000)
Stock Repurchased and Retired During Period, Value	(314)	$ (314)
Cash dividends paid on common stock	$ (30,892)		(30,892)
Treasury Stock, Shares, Acquired	(624,158)	(624,000)
Treasury Stock, Value, Acquired, Cost Method	$ (21,863)				(21,863)
Balance, value at Jun. 30, 2019	$ 2,133,638	$ 1,644,922	$ 463,429	$ 47,150	$ (21,863)
Shares, Outstanding		72,924,000