Outstanding Common Stock Equivalents That Were Anti-Dilutive Tickmarks (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Anti-dilutive Non-Vested Stock Weighted Average Grant Date Fair Value	$ 38.40	$ 52.07	$ 61.15