Consolidated Statements of Changes in Shareholders' Equity - USD ($) $ in Thousands	Total	Preferred Stock [Member]	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]
Balance, value at Dec. 31, 2014	$ 1,228,175	$ 2,217	$ 985,839	$ 234,498	$ 5,621
Balance (in shares) at Dec. 31, 2014		9,000	57,437,000
Net income	98,827			98,827
Other comprehensive income	(11,916)				(11,916)
Issuance of common stock - stock option and other plans, shares			49,000
Issuance of common stock - stock option and other plans, value	1,258		$ 1,258
Issuance of common stock - restricted stock awards, net of canceled awards, shares			270,000
Issuance of common stock - restricted stock awards, net of canceled awards, value	4,090		$ 4,090
Purchase and retirement of common stock (in shares)			(32,000)
Purchase and retirement of common stock, value	(906)		$ (906)
Dividends, Preferred Stock, Cash	(137)			(137)
Cash dividends paid on common stock	(77,263)			(77,263)
Balance, value at Dec. 31, 2015	$ 1,242,128	$ 2,217	$ 990,281	255,925	(6,295)
Balance (in shares) at Dec. 31, 2015		9,000	57,724,000
Preferred Stock, Dividends, Per Share, Cash Paid	$ 1.34
Common Stock, Dividends, Per Share, Cash Paid	$ 1.34
Net income	$ 104,866			104,866
Other comprehensive income	(12,704)				(12,704)
Stock Adjustment Value Deferred Compensation	(11)		$ (11)
Issuance of common stock - stock option and other plans, shares			50,000
Issuance of common stock - stock option and other plans, value	1,349		$ 1,349
Issuance of common stock - restricted stock awards, net of canceled awards, shares			306,000
Issuance of common stock - restricted stock awards, net of canceled awards, value	5,009		$ 5,009
Tax benefit deficiency associated with share-based compensation	334		$ 334
Purchase and retirement of common stock (in shares)			(38,000)
Purchase and retirement of common stock, value	(1,125)		$ (1,125)
Dividends, Preferred Stock, Cash	(157)			(157)
Cash dividends paid on common stock	(88,677)			(88,677)
Balance, value at Dec. 31, 2016	$ 1,251,012	$ 2,217	$ 995,837	271,957	(18,999)
Balance (in shares) at Dec. 31, 2016		9,000	58,042,000
Preferred Stock, Dividends, Per Share, Cash Paid	$ 1.53
Common Stock, Dividends, Per Share, Cash Paid	$ 1.53
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2016-09 [Member]	$ 67		$ 184	(117)
Net income	112,828			112,828
Other comprehensive income	856				856
Issuance of common stock, net of offering costs, shares		0	14,642,000
Issuance of common stock, net of offering costs, value	636,385	$ 0	$ 636,385
Stock Adjustment Value Deferred Compensation	$ 1		$ 1
Issuance of common stock - stock option and other plans, shares	2,242		49,000
Issuance of common stock - stock option and other plans, value	$ 1,980		$ 1,980
Issuance of common stock - restricted stock awards, net of canceled awards, shares			241,000
Issuance of common stock - restricted stock awards, net of canceled awards, value	7,745		$ 7,745
Conversion of Stock, Shares Converted		(9,000)
Conversion of Stock, Amount Converted		$ (2,217)
Conversion of Stock, Shares Issued			102,000
Conversion of Stock, Amount Issued	0		$ 2,217
Purchase and retirement of common stock (in shares)			(56,000)
Purchase and retirement of common stock, value	(2,299)		$ (2,299)
Stock Redeemed or Called During Period, Value	(7,345)		(7,345)
Cash dividends paid on common stock	(51,308)			(51,308)
Balance, value at Dec. 31, 2017	$ 1,949,922	$ 0	$ 1,634,705	337,442	(22,225)
Balance (in shares) at Dec. 31, 2017		0	73,020,000
Preferred Stock, Dividends, Per Share, Cash Paid	$ 0.00
Common Stock, Dividends, Per Share, Cash Paid	$ 0.88
Cumulative Effect of New Accounting Principle in Period of Adoption | Accounting Standards Update 2018-02 [Member]	$ 0			$ 4,082	$ (4,082)