Borrowings - Schedule of Borrowings (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Debt Facilities [Line Items]
Carrying Value	$ 923.2	$ 886.3
Borrowing Outstanding	925.0	886.3
Revolving Credit Facility [Member]
Debt Facilities [Line Items]
Borrowing Outstanding		386.3
Carrying Value		386.3
Term Loan [Member]
Debt Facilities [Line Items]
Borrowing Outstanding	25.0	500.0
Carrying Value	25.0	500.0
3.875% senior notes due 2023 [Member]
Debt Facilities [Line Items]
Carrying Value	499.8
Borrowing Outstanding	500.0
5.625% senior notes due 2043 [Member]
Debt Facilities [Line Items]
Borrowing Outstanding	400.0
Carrying Value	$ 398.4