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Common stock (Details) - USD ($) $ / shares in Units, $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Dividends declared per common share | $ 0.8350 | $ 0.8150 | $ 0.7950
Dividends declared to common stockholders | $ 168,489 | $ 162,408 | $ 156,453
Credit agreement limitation on company's subsidiaries ratio of funded debt to capitalization | 65.00% |  |
Company's subsidiaries net assets restricted from use for dividend payments | $ 1,400,000 |  |
Common stock |  |  |
Dividends declared to common stockholders | $ 167,400 | $ 162,100 | $ 155,700
Centennial [Member] |  |  |
Maximum distributions to the company as a ratio of average consolidated indebtedness to consolidated EBITDA | 350.00% |  |

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- Definition The maximum distributions to the company by the subsidiary as a ratio of average consolidated indebtedness, as of the last day of fiscal quarter and the three previous quarters, to consolidated EBITDA. + References No definition available. + Details
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