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		73,249
Adjustment to opening retained earnings, net of tax, pursuant to adoption of ASU | Accounting Standards Update 2016-02 [Member]	$ 782		$ 782
Common Stock, Dividends, Per Share, Cash Paid	$ 0.84
Balance, value at Dec. 31, 2018	$ 2,033,649	$ 1,642,246	426,708	$ (35,305)
Net income	97,595		97,595
Other comprehensive income (loss)	82,455			82,455
Issuance of common stock - stock option and other plans, shares		27
Issuance of common stock - stock option and other plans, value	$ 893	$ 893
Stock Adjustment Deferred Compensation - Shares	0
Stock Adjustment Value Deferred Compensation	$ 1
Issuance of common stock - restricted stock awards, net of canceled awards, shares		345
Issuance of common stock - restricted stock awards, net of canceled awards, value	4,528	$ 4,528
Purchase and retirement of common stock, Shares		(73)
Purchase and retirement of common stock, Value	(2,746)	$ (2,746)
Cash dividends declared on common stock	(61,656)		(61,656)
Purchase of treasury stock, Shares		(624)
Purchase of treasury stock, value	(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
Common Stock, Dividends, Per Share, Cash Paid	$ 0.42
Balance, value at Mar. 31, 2019	$ 2,088,620	$ 1,642,977	442,597	3,046
Net income	51,724		51,724
Other comprehensive income (loss)	44,104			44,104
Issuance of common stock - stock option and other plans, shares		2
Issuance of common stock - stock option and other plans, value	$ 15	$ 15
Stock Adjustment Deferred Compensation - Shares	0
Stock Adjustment Value Deferred Compensation	$ 1
Issuance of common stock - restricted stock awards, net of canceled awards, shares		(10)
Issuance of common stock - restricted stock awards, net of canceled awards, value	2,243	$ 2,243
Purchase and retirement of common stock, Shares		(9)
Purchase and retirement of common stock, Value	(314)	$ (314)
Cash dividends declared on common stock	(30,892)		(30,892)
Purchase of treasury stock, Shares		(624)
Purchase of treasury stock, value	(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
Shares, Outstanding		72,124
Adjustment to opening retained earnings, net of tax, pursuant to adoption of ASU | Accounting Standards Update 2016-13 [Member]	$ (2,500)		(2,457)
Common Stock, Dividends, Per Share, Cash Paid	$ 0.78
Balance, value at Dec. 31, 2019	$ 2,159,962	$ 1,650,753	519,676	40,367	(50,834)
Net income	51,210		51,210
Other comprehensive income (loss)	140,710			140,710
Issuance of common stock - stock option and other plans, shares		26
Issuance of common stock - stock option and other plans, value	945	$ 945
Stock Adjustment Deferred Compensation - Shares		0
Stock Adjustment Value Deferred Compensation	4	$ 4
Issuance of common stock - restricted stock awards, net of canceled awards, shares		234
Issuance of common stock - restricted stock awards, net of canceled awards, value	4,909	$ 4,909
Purchase and retirement of common stock, Shares		(67)
Purchase and retirement of common stock, Value	(2,482)	$ (2,482)
Cash dividends declared on common stock	$ (56,046)		(56,046)
Purchase of treasury stock, Shares	(731)	(731)
Purchase of treasury stock, value	$ (20,000)				(20,000)
Balance, value at Jun. 30, 2020	$ 2,276,755	$ 1,654,129	512,383	181,077	(70,834)
Shares, Outstanding		71,575
Common Stock, Dividends, Per Share, Cash Paid	$ 0.28
Balance, value at Mar. 31, 2020	$ 2,213,602	$ 1,651,399	495,830	137,207	(70,834)
Net income	36,582		36,582
Other comprehensive income (loss)	43,870			43,870
Issuance of common stock - stock option and other plans, shares		0
Issuance of common stock - stock option and other plans, value	0	$ 0
Stock Adjustment Deferred Compensation - Shares		0
Stock Adjustment Value Deferred Compensation	1	$ 1
Issuance of common stock - restricted stock awards, net of canceled awards, shares		12
Issuance of common stock - restricted stock awards, net of canceled awards, value	2,737	$ 2,737
Purchase and retirement of common stock, Shares		1
Purchase and retirement of common stock, Value	(8)	$ (8)
Cash dividends declared on common stock	(20,029)		(20,029)
Balance, value at Jun. 30, 2020	$ 2,276,755	$ 1,654,129	$ 512,383	$ 181,077	$ (70,834)
Shares, Outstanding		71,586