Equity Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended			6 Months Ended		55 Months Ended
	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Dec. 31, 2014
Equity [Abstract]
Repurchases of common shares				212,588	66,913	788,029
Weighted average purchase price per share				$ 131.67	$ 186.77	$ 134.09
Total cost of treasury shares acquired				$ 28,000	$ 12,500	$ 105,700
Common stock available to repurchase	411,971			411,971		411,971
Cash dividends declared per share	$ 0.45	$ 0.45	$ 0.45	$ 0.90	$ 0.45
Dividends payable	$ 4,251			$ 4,251		$ 4,251	$ 4,270
Total cash dividends				$ 8,143	$ 4,179
Dividend, to be paid date				Aug. 14, 2015
Dividend, record date				Jul. 31, 2015