Common stock (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
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%

Company's (excluding its subsidiaries) net assets restricted from use for dividend payments.

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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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- Definition

Ratio of earnings before interest and taxes to interest expense as specified in debt convenants.

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No definition available.

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- Definition

Aggregate dividends declared during the period for each share of common stock outstanding.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

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- Definition

Amount of paid and unpaid common stock dividends declared with the form of settlement in cash.

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