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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: 12,373 – 2018; 15,419 – 2017) |  | 31 | 693 |  |  |
Stock purchase and compensation plans (shares: 433,009 – 2018; 515,011 – 2017) |  | 1,030 | 11,994 |  |  |
Net income | $ 91,866 |  |  | 91,866 |  |
Dividends to stockholders ($0.36 per share – 2018; $0.32 per share – 2017) |  |  |  | (18,927) |  |
Other comprehensive (loss) income | 40,447 |  |  |  | 40,447 |
Acquisition of treasury stock (shares: 105,452 – 2018; 134,910 – 2017) |  |  |  |  |  | (5,948)
End of period at Jun. 30, 2017 | 1,652,556 | 204,302 | 359,982 | 1,641,820 | 24,497 | (578,045)
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: 12,373 – 2018; 15,419 – 2017) |  | 25 | 686 |  |  |
Stock purchase and compensation plans (shares: 433,009 – 2018; 515,011 – 2017) |  | 866 | 13,238 |  |  |
Net income | 77,744 |  |  | 77,744 |  |
Dividends to stockholders ($0.36 per share – 2018; $0.32 per share – 2017) |  |  |  | (21,448) |  |
Other comprehensive (loss) income | (79,668) |  |  |  | (79,668) |
Acquisition of treasury stock (shares: 105,452 – 2018; 134,910 – 2017) |  |  |  |  |  | (6,245)
End of period at Jun. 30, 2018 | $ 1,698,155 | $ 205,460 | $ 381,641 | 1,779,928 | (84,517) | $ (584,357)
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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