Nature of Business and Financial Statement Presentation - Non-cash Investing and Financing Activities (Detail) - USD ($) $ in Thousands	6 Months Ended
	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