Investments - Contractual Maturities of Fixed Maturity Securities and Short Term Investments Classified as Available for Sale (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Due in one year or less	73.10%	37.90%
Due after one year through five years	23.60%	53.00%
Due after five years through ten years		0.60%
Due after ten years	1.10%	0.50%
Total percent of fair value	97.80%	92.00%
Total percent of fair value	100.00%	100.00%
Due in one year or less	$ 227,225	$ 230,377
Due after one year through five years	73,299	322,131
Due after five years through ten years		3,367
Due after ten years	3,429	2,894
Total fair value with single maturity date	303,953	558,769
Total fair value	310,642	607,316
Due in one year or less	225,146	230,550
Due after one year through five years	68,264	308,062
Due after five years through ten years		3,296
Due after ten years	3,083	2,990
Total amortized cost with single maturity date	296,493	544,898
Total amortized cost	302,954	590,588
Residential mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Total percent of fair value	1.60%
Total percent of fair value		2.30%
Total fair value	4,891	13,712
Total amortized cost	4,701	13,544
Commercial mortgage-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Total percent of fair value	0.50%
Total percent of fair value		2.60%
Total fair value	1,421	15,717
Total amortized cost	1,376	12,680
Asset-backed [Member]
Schedule of Available-for-sale Securities [Line Items]
Total percent of fair value	0.10%
Total percent of fair value		3.10%
Total fair value	377	19,118
Total amortized cost	$ 384	$ 19,466