Investments - Additional Information (Detail) (USD $)	3 Months Ended								12 Months Ended					24 Months Ended	12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2012 Settlement Agreement	Dec. 31, 2011 Settlement Agreement	Dec. 31, 2012 Settlement Agreement	Dec. 31, 2012 General Decline in Market Yield	Dec. 31, 2011 General Decline in Market Yield	Dec. 31, 2012 Mortgage-backed and asset-backed securities Y	Dec. 31, 2012 Non-government backed securities Maximum	Dec. 31, 2011 Non-government backed securities Maximum	Dec. 31, 2012 Non-government backed securities Investments	Dec. 31, 2011 Non-government backed securities Investments	Dec. 31, 2012 Equity Securities Maximum
Schedule of Investments [Line Items]
Non-credit OTTI on debt securities included in other comprehensive income					$ 1,700,000					$ 1,700,000
Non-credit OTTI on debt securities in an unrealized gain position									20,000
Securities, effective maturities																	4.0
Gross Unrealized Loss																				200,000	1,400,000
Percentage of fair value over cost																						80.00%
Annualized pre-tax investment yield									2.40%	3.00%	3.50%				2.70%	3.20%
Portfolio duration of investments									3 years 7 months 6 days	3 years 7 months 6 days	4 years 4 months 24 days
Investment Expenses									10,386,000	7,595,000	2,327,000	4,500,000	4,700,000	9,200,000
Investment performance fee									2,800,000
Net realized gains from sale of investments	30,254,000	4,761,000	4,217,000	1,842,000	7,141,000	3,238,000	3,006,000	(1,389,000)	41,074,000	11,996,000	41,319,000
Total Restricted Net Assets	$ 519,201,000				$ 462,459,000				$ 519,201,000	$ 462,459,000
Percentage of investment																		5.00%	5.00%