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Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional paid-in capital [Member]	Retained Earnings [Member]	Accumulated other comprehensive income (loss) [Member]	Treasury stock [Member]
Balance at beginning of year, as adjusted				$ 1,723,632	$ (4,849)
Beginning of year at Dec. 31, 2017		$ 204,569	$ 367,717	1,698,613	20,170	$ (578,112)
Dividend reinvestment plan (shares: 5,694 – 2019; 6,298 – 2018)		13	353
Stock purchase and compensation plans (shares: 371,628 – 2019; 349,255 – 2018)		698	7,085
Net income	$ 18,925			18,925
Dividends to stockholders ($0.20 per share – 2019; $0.18 per share – 2018)				(10,725)
Other comprehensive income (loss)	(63,394)				(63,394)
Acquisition of treasury stock (shares: 102,971 – 2019; 103,170 – 2018)						(6,116)
End of period at Mar. 31, 2018	1,659,796	205,280	375,155	1,731,832	(68,243)	(584,228)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01 [Member]				30,726	(30,726)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2018-02 [Member]				(5,707)	5,707
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-02 [Member]				0
Balance at beginning of year, as adjusted				1,858,756	(77,956)
Beginning of year at Dec. 31, 2018	1,791,802	205,697	390,315	1,858,414	(77,956)	(584,668)
Dividend reinvestment plan (shares: 5,694 – 2019; 6,298 – 2018)		11	365
Stock purchase and compensation plans (shares: 371,628 – 2019; 349,255 – 2018)		743	8,201
Net income	61,348			61,348
Dividends to stockholders ($0.20 per share – 2019; $0.18 per share – 2018)				(11,985)
Other comprehensive income (loss)	80,980				80,980
Acquisition of treasury stock (shares: 102,971 – 2019; 103,170 – 2018)						(6,585)
End of period at Mar. 31, 2019	$ 1,925,222	$ 206,451	$ 398,881	1,908,119	3,024	$ (591,253)
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-01 [Member]				0	0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2018-02 [Member]				0	$ 0
Cumulative effect of new accounting principle in period of adoption | Accounting Standards Update 2016-02 [Member]				$ 342

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Amount after tax of increase (decrease) to equity or (increase) decrease to net assets, resulting from the cumulative effect adjustment of a new accounting principle applied in the period of adoption.

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