Shareholders' Equity - Schedule of Repurchase of Shares (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended				13 Months Ended
	Dec. 31, 2012		Dec. 31, 2011	Dec. 31, 2010	Jan. 31, 2013
Schedule Of Share Repurchase Programs [Line Items]
Number of shares repurchased	1,488,308		1,607,745	3,217,561	1,659,788
Average Price of Shares Repurchased	$ 33.38
Total Cost					$ 50.6
12/14/2011 [Member]
Schedule Of Share Repurchase Programs [Line Items]
Date Trading Plan Initiated	Dec. 14, 2011
2012 Purchase Period	01/01/12-02/16/12	[1]
Trading plans purchase period date range start	01/01/12
Trading plans purchase period date range end	02/16/12
Number of shares repurchased	184,311
Average Price of Shares Repurchased	$ 29.13
Total Cost	5.4
Repurchase Authorization Year	2011
03/14/2012 [Member]
Schedule Of Share Repurchase Programs [Line Items]
Date Trading Plan Initiated	Mar. 14, 2012
2012 Purchase Period	03/16/12-05/10/12
Trading plans purchase period date range start	03/16/12
Trading plans purchase period date range end	05/10/12
Number of shares repurchased	468,944
Average Price of Shares Repurchased	$ 29.32
Total Cost	13.8
Repurchase Authorization Year	2011
06/14/2012 [Member]
Schedule Of Share Repurchase Programs [Line Items]
Date Trading Plan Initiated	Jun. 14, 2012
2012 Purchase Period	06/18/12-08/09/12
Trading plans purchase period date range start	06/18/12
Trading plans purchase period date range end	08/09/12
Number of shares repurchased	332,082
Average Price of Shares Repurchased	$ 29.63
Total Cost	9.8
Repurchase Authorization Year	2011
12/14/2012 [Member]
Schedule Of Share Repurchase Programs [Line Items]
Date Trading Plan Initiated	Dec. 14, 2012
2012 Purchase Period	12/18/12-01/17/13	[2]
Trading plans purchase period date range start	12/18/12
Trading plans purchase period date range end	02/28/13
Number of shares repurchased	289,143
Average Price of Shares Repurchased	$ 34.58
Total Cost	$ 9.9
Repurchase Authorization Year	2011

[1] The above table only reflects the 2012 activity under this Rule 10b5-1 trading plan. In 2011, 73,018 shares were repurchased under this Rule 10b5-1 trading plan for a total cost of $2.1 million. Total shares repurchased in 2011 and 2012 under this Rule 10b5-1 trading plan are 257,329 shares at an average price of $29.06 for a total cost of $7.5 million.

[2] Through December 31, 2012, 117,663 shares were repurchased at an average price of $33.38 for a total cost of $3.9 million.