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Common Equity and Earnings per Share - Additional Information (Detail) - USD ($)	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Class of Stock
Payments of dividends, common stock			$ 0		$ 0
Preferred stock, shares issued	0		0		0
Preferred stock, shares outstanding	0		0		0
Class A
Class of Stock
Common stock, votes per share			one
Class A \| 2013 Plan
Class of Stock
Shares issuable under stock options excluded from calculation of diluted earnings per share	979,000	412,000	880,000	385,000
Class B Convertible
Class of Stock
Common stock, votes per share			ten