Common Equity and Earnings per Share - Additional Information (Detail) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Class of Stock
Payments of dividends, common stock	$ 0		$ 0
Preferred stock, shares issued	0		0
Preferred stock, shares outstanding	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	345,000	377,000
Class B Convertible
Class of Stock
Common stock, votes per share	Ten