Equity Transactions (Detail) - USD ($)	3 Months Ended							12 Months Ended			63 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Oct. 21, 2015
Equity [Abstract]
Repurchase of common shares (in shares)								638,703	225,441		1,214,144
Weighted average price (in $ per share)								$ 125.22	$ 178.54		$ 129.85
Cost of shares repurchased								$ 80,000,000	$ 40,300,000		$ 157,700,000
Shares authorized for repurchase (in shares)												1,500,000
Remaining shares authorized for repurchase (in shares)	1,485,856							1,485,856			1,485,856
Cash dividends declared (in $ per share)	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 0.45	$ 1.8	$ 1.35	$ 0
Cash dividends declared								$ 16,009,000	$ 12,451,000	$ 0
Dividends payable	$ 4,233,000				$ 4,270,000			$ 4,233,000	$ 4,270,000	$ 0	$ 4,233,000
Dividend, to be paid date								Feb. 12, 2016
Dividend, record date								Jan. 29, 2016