Investments and Marketable Securities - Aggregate Market Value and Unrealized Losses of Investments (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Number of investments, Unrealized Loss Less 12 months	56	56
Aggregate Market Value, Unrealized Loss Less 12 months	$ 45,577	$ 60,616
Unrealized Losses, Unrealized Loss Less 12 months	(53)	(865)
Number of investments, Unrealized Loss Greater 12 months	16	70
Aggregate Market Value, Unrealized Loss Greater 12 months	10,144	42,377
Unrealized Losses, Unrealized Loss Greater 12 months	(446)	(572)
Government Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of investments, Unrealized Loss Less 12 months	4	5
Aggregate Market Value, Unrealized Loss Less 12 months	5,179	4,599
Unrealized Losses, Unrealized Loss Less 12 months	(3)	(3)
Number of investments, Unrealized Loss Greater 12 months	1	6
Aggregate Market Value, Unrealized Loss Greater 12 months	1,790	6,708
Unrealized Losses, Unrealized Loss Greater 12 months	0	(4)
State And Municipal Bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of investments, Unrealized Loss Less 12 months	19	16
Aggregate Market Value, Unrealized Loss Less 12 months	24,969	24,556
Unrealized Losses, Unrealized Loss Less 12 months	(27)	(7)
Number of investments, Unrealized Loss Greater 12 months	1	11
Aggregate Market Value, Unrealized Loss Greater 12 months	1,092	18,612
Unrealized Losses, Unrealized Loss Greater 12 months	(1)	(36)
Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Number of investments, Unrealized Loss Less 12 months	33	35
Aggregate Market Value, Unrealized Loss Less 12 months	15,429	31,461
Unrealized Losses, Unrealized Loss Less 12 months	(23)	(855)
Number of investments, Unrealized Loss Greater 12 months	14	53
Aggregate Market Value, Unrealized Loss Greater 12 months	7,262	17,057
Unrealized Losses, Unrealized Loss Greater 12 months	$ (445)	$ (532)