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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				$ 1,568,881	$ (15,950)
Beginning of year at Dec. 31, 2016		$ 203,241	$ 347,295	1,568,881	(15,950)	$ (572,097)
Dividend reinvestment plan (shares: 17,683 – 2018; 22,278 – 2017)		45	1,025
Stock purchase and compensation plans (shares: 484,491 – 2018; 537,588 – 2017)		1,075	14,417
Net income	$ 138,584			138,584
Dividends to stockholders ($0.54 per share – 2018; $0.48 per share – 2017)				(28,424)
Other comprehensive (loss) income	47,265				47,265
Acquisition of treasury stock (shares: 108,311 – 2018; 136,012 – 2017)						(6,005)
End of period at Sep. 30, 2017	1,699,352	204,361	362,737	1,679,041	31,315	(578,102)
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
Balance at beginning of year, as adjusted				1,723,632	(4,849)
Beginning of year at Dec. 31, 2017	1,712,957	204,569	367,717	1,698,613	20,170	(578,112)
Dividend reinvestment plan (shares: 17,683 – 2018; 22,278 – 2017)		35	1,013
Stock purchase and compensation plans (shares: 484,491 – 2018; 537,588 – 2017)		969	16,721
Net income	133,179			133,179
Dividends to stockholders ($0.54 per share – 2018; $0.48 per share – 2017)				(32,204)
Other comprehensive (loss) income	(87,727)				(87,727)
Acquisition of treasury stock (shares: 108,311 – 2018; 136,012 – 2017)						(6,423)
End of period at Sep. 30, 2018	$ 1,738,520	$ 205,573	$ 385,451	1,824,607	(92,576)	$ (584,535)
Cumulative effect of new accounting principle in period of adoption					(25,019)
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

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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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