Stockholder's Equity - Additional Information (Detail) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Stockholders' Equity Note [Abstract]
Common stock reserved and available for issuance	1,500,000
Common stock shares available for grant	278,564
Percentage of directors' stock grant vested on one year anniversary date	100.00%
Granted, Weighted average grant date fair value | $ / shares	$ 61.07
Total unrecognized compensation cost related to nonvested shares of common stock granted | $	$ 21,400
Weighted average period of cost recognized	3 years 5 months 4 days
Total fair value of shares of common stock vested | $	$ 5,600
Compensation expense related to nonvested stock grants | $	$ 1,600
Price of shares issued under Employee Stock Purchase Plan as a percentage of FMV	95.00%
Number of shares reserved for the Employee Stock Purchase Plan	250,000
Additional shares authorized for issuance	250,000
Shares redeemed to satisfy personal tax obligations	56,772
Treasury shares redeemed to pay income tax | $	$ 3,467
Employee
Stockholders' Equity Note [Abstract]
Nonvested stock grants to employees	156,345
Employee Period of Vested Shares	5 years
Director
Stockholders' Equity Note [Abstract]
Nonvested stock grants to Independent directors	13,600
Director Period of Vested Shares	1 year