Equity Transactions - Additional Information (Detail) (USD $)	3 Months Ended			12 Months Ended		49 Months Ended	1 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2014	Sep. 30, 2013	Dec. 10, 2014
Equity, Class of Treasury Stock [Line Items]
Treasury stock, shares acquired				225,441	105,000	575,441
Weighted average purchase price per share				$ 178.54	$ 187.61	$ 134.98
Total cost of treasury shares acquired				$ 40,300,000	$ 19,700,000	$ 77,700,000
Repurchased additional shares of common stock, shares								500,000
Common stock remaining authorized for repurchase	624,559			624,559		624,559
Total cash dividends				12,451,000	0
Cash dividends declared per share	$ 0.45	$ 0.45	$ 0.45	$ 1.35
Dividend, record date				Jan. 30, 2015
Dividend, to be paid date				Feb. 13, 2015
Dividends payable	4,270,000			4,270,000		4,270,000
Common stock value					191,578,000
Public Offering [Member]
Equity, Class of Treasury Stock [Line Items]
Common stock, shares issued							1,300,000
Common stock value							$ 191,800,000