Stock-Based Compensation and Stock Awards - Schedule of DSA Activity (Detail) - DSA	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Number of shares
Nonvested options, Beginning Balance (in shares) | shares	13,422
Granted (in shares) | shares	9,340
Vested (in shares) | shares	(5,248)
Forfeited (in shares) | shares	0
Nonvested options, Ending Balance (in shares) | shares	17,514
Weighted Average Grant Date Fair Value Per Unit
Nonvested Weighted Average Grant Date Fair Value, Beginning Balance (in dollars per share) | $ / shares	$ 62.46
Granted (in dollars per share) | $ / shares	51.58
Vested (in dollars per share) | $ / shares	68.70
Forfeited (in dollars per share) | $ / shares	0.00
Nonvested Weighted Average Grant Date Fair Value, Ending Balance (in dollars per share) | $ / shares	$ 54.80