Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Preferred Stock [Member]
Balance, value at Dec. 31, 2015	$ 1,242,128	$ 990,281	$ 255,925	$ (6,295)	$ 2,217
Balance, shares at Dec. 31, 2015		57,724,000			9,000
Net income	74,148		74,148
Other comprehensive income	22,800			22,800
Issuance of common stock - stock option and other plans, value	1,304	$ 1,304
Issuance of common stock - stock option and other plans, shares		47,000
Issuance of common stock - restricted stock awards, net of cancelled awards, value	3,626	$ 3,626
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		310,000
Stock Repurchased and Retired During Period, Value	(1,113)	$ (1,113)
Stock Repurchased During Period, Shares		(38,000)
Dividends, Preferred Stock, Cash	(117)		(117)
Cash dividends paid on common stock	(66,041)		(66,041)
Balance, value at Sep. 30, 2016	1,276,735	$ 994,098	263,915	16,505	$ 2,217
Balance, shares at Sep. 30, 2016		58,043,000			9,000
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		58,042,000			8,782
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		49,000
Issuance of common stock - restricted stock awards, net of cancelled awards, value	5,915	$ 5,915
Conversion of Stock, Shares Converted					(9,000)
Conversion of Stock, Amount Converted					$ (2,217)
Conversion of Stock, New Issuance		102,000
Conversion of Stock, Amount Issued	0	$ 2,217
Issuance of common stock - restricted stock awards, net of cancelled awards, shares		238,000
Stock Repurchased and Retired During Period, Value	(2,246)	$ (2,246)
Stock Repurchased During Period, Shares		(55,000)
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		58,376,000			0