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	97,100		97,100
Other comprehensive income	13,066			13,066
Issuance of common stock - stock option and other plans, value	1,980	$ 1,980
Issuance of common stock - stock option and other plans, shares		49
Issuance of common stock - restricted stock awards, net of cancelled awards, value	5,915	$ 5,915
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		238
Stock Repurchased and Retired During Period, Value	(2,246)	$ (2,246)
Stock Repurchased During Period, Shares		(55)
Cash dividends paid on common stock	(38,466)		(38,466)
Balance, value at Sep. 30, 2017	1,328,428	$ 1,003,887	330,474	(5,933)	$ 0
Balance, shares at Sep. 30, 2017		58,376			0
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)	$ 0
Balance, shares at Dec. 31, 2017		73,020			0
Net income	128,134		128,134
Other comprehensive income	(47,941)			(47,941)
Issuance of common stock - stock option and other plans, value	1,857	$ 1,857
Issuance of common stock - stock option and other plans, shares		45
Issuance of common stock - restricted stock awards, net of cancelled awards, value	6,231	$ 6,231
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		257
Stock Repurchased and Retired During Period, Value	(2,660)	$ (2,660)
Stock Repurchased During Period, Shares		(62)
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
Balance, shares at Sep. 30, 2018		73,260			0
Stock Adjustment Deferred Compensation - Shares		0
Stock Adjustment Value Deferred Compensation	$ 7	$ 7