Dividends - Additional Information (Detail)	9 Months Ended
	Sep. 30, 2017 USD ($)	Dec. 31, 2016
Dividends [Abstract]
Target percentage used to determine annual variable dividend	33.33%	33.33%
Gainshare factor used to determine annual variable dividend range, minimum	0
Minimum Payments of Ordinary Dividends, Common Stock	$ 0
Gainshare factor used to determine annual variable dividend range, maximum	2
Gainshare factor used to determine annual variable dividend	1.67