Investments in Marketable Debt Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Marketable securities sales and maturities	$ 628.5	$ 445.8
Gross realized gains from sales of marketable securities	1.7	1.9
Gross realized loss from sales of marketable securities	0.5	0.1
Marketable debt securities in an unrealized loss position for greater than 12 months	1.0		0
Marketable debt securities in an unrealized loss position for greater than 12 months, unrealized losses			0
Marketable debt securities in an unrealized loss position for less than 12 months	246.1		291.0
Marketable debt securities in an unrealized loss position for less than 12 months, unrealized losses	0.7		0.4
Maximum
Schedule of Available-for-sale Securities [Line Items]
Marketable debt securities in an unrealized loss position for greater than 12 months, unrealized losses	$ 0.1