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Nature of Business and Financial Statement Presentation - Non-cash Investing and Financing Activities (Detail) - USD ($) $ in Thousands | 6 Months Ended |
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Jun. 30, 2017 | Jun. 30, 2016 | Dec. 31, 2016
Organization Consolidation And Presentation Of Financial Statements [Abstract] |  |  |
Accounts payable related to construction in progress | $ 15,700 | $ 20,800 |
Dividends declared | $ 76,744 | $ 75,100 | $ 75,245

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