Treasury Stock and Dividends - Additional Information (Detail) (USD $)	0 Months Ended	3 Months Ended		6 Months Ended		43 Months Ended
	May 15, 2014	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014
Equity [Abstract]
Treasury stock, shares acquired				66,913	80,000	416,913
Weighted average purchase price per share				$ 186.77	$ 187.03	$ 119.74
Total cost of treasury shares acquired				$ 12,500,000	$ 15,000,000	$ 49,900,000
Common stock remaining authorized for repurchase		283,087		283,087		283,087
Cash dividends declared per share	$ 0.45	$ 0.45	$ 0.00	$ 0.45	$ 0.00
Dividend, record date				Jul. 31, 2014
Dividend, to be paid date				Aug. 14, 2014
Dividends payable		$ 4,179,000	$ 0	$ 4,179,000	$ 0	$ 4,179,000