Investments in Marketable Debt Securities - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Gross realized gains from sales of marketable securities	$ 2.0	$ 3.8	$ 3.2
Gross realized loss from sales of marketable securities	0.7	0.3	1.3
Variable Rate Demand Obligation
Schedule of Available-for-sale Securities [Line Items]
Marketable debt securities	$ 0.4