Stock-Based Compensation - Summary of Restricted Common Shares (Detail) - Restricted Stock And Units [Member]	12 Months Ended
	Dec. 31, 2017 $ / shares shares
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Non-vested Number, Beginning Balance (in shares) | shares	371,559
Granted (in shares) | shares	118,562
Vested (in shares) | shares	(104,608)
Forfeited (in shares) | shares	(31,466)
Non-vested Number, Ending Balance (in shares) | shares	354,047
Non-vested Weighted Average Grant Date Fair Value, Beginning Balance (in dollars per share) | $ / shares	$ 63.98
Granted (in dollars per share) | $ / shares	107.57
Vested (in dollars per share) | $ / shares	67.01
Forfeited (in dollars per share) | $ / shares	71.29
Non-vested Weighted Average Grant Date Fair Value, Ending Balance (in dollars per share) | $ / shares	$ 77.03