Investments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended			6 Months Ended
	Jun. 30, 2013 Securities	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2013 Securities	Jun. 30, 2012	Dec. 31, 2012 Securities
Investment [Line Items]
Number of securities accounted for gross unrealized losses	158			158		49
Cost basis of securities at disposal	$ 5.0	$ 0.9		$ 5.0	$ 20.0
Realized gains on sale of equity securities	0.6	0.1		0.6	1.9
Impairment losses recognized		0	0
Equity Securities [Member]
Investment [Line Items]
Impairment losses recognized	$ 1.9			$ 1.9