Investments - Additional Information (Detail) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Securities	Sep. 30, 2012	Sep. 30, 2013 Securities	Sep. 30, 2012	Dec. 31, 2012 Securities
Investment [Line Items]
Other investments	$ 10,271,000		$ 10,271,000
Number of securities accounted for gross unrealized losses	158		158		49
Cost basis of securities at disposal	1,700,000	11,100,000	6,700,000	31,200,000
Realized gains on sale of equity securities		900,000	200,000	2,700,000
Realized loss on sale of equity securities	500,000
Impairment losses recognized				0
Equity Securities [Member]
Investment [Line Items]
Impairment losses recognized			$ 2,200,000