Equity Transactions - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended				6 Months Ended		67 Months Ended
	Jun. 30, 2016	Mar. 31, 2016	Jun. 30, 2015	Mar. 31, 2015	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016
Equity, Class of Treasury Stock [Line Items]
Repurchases of common shares (in shares)					732,713	212,588
Weighted average purchase price per share (in $ per share)					$ 83.12	$ 131.67
Total cost of treasury shares acquired					$ 62,100	$ 28,000
Cash dividends declared per share (in $ per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.90	$ 0.9
Total cash dividends					$ 7,452	$ 8,143
Repurchase Plan, Tender Offer [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchases of common shares (in shares)					556,509
Weighted average purchase price per share (in $ per share)					$ 82.50
Total cost of treasury shares acquired					$ 47,100
Repurchase Plan, Excluding Tender Offer [Member]
Equity, Class of Treasury Stock [Line Items]
Repurchases of common shares (in shares)							1,390,348
Weighted average purchase price per share (in $ per share)							$ 124.17
Total cost of treasury shares acquired							$ 172,700
Common stock available to repurchase (in shares)	1,309,652				1,309,652		1,309,652
Additional Paid-in Capital [Member]
Equity, Class of Treasury Stock [Line Items]
Total cash dividends					$ 7,452	$ 8,143
Additional Paid-in Capital [Member] | Repurchase Plan, Tender Offer [Member]
Equity, Class of Treasury Stock [Line Items]
Shares repurchased and retired					$ 47,100