Reinsurance Agreements (Details) (USD $)	3 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2013 Structure	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2012
Reinsurance Agreements [Abstract]
Number of types of reinsurance structures	3
Liability for Catastrophe Claims [Line Items]
Prepaid reinsurance premiums	$ 4,063,000			$ 7,045,000
Increase in total insured value		2,100,000,000	172,200,000
Percentage increase in total insured value (in hundredths)		12.60%	1.40%
Increase reinsurance premiums for total insured value		2,400,000	500,000
Percentage increase in reinsurance premiums for total insured value (in hundredths)		3.00%	1.10%
Increase in probable maximum loss			63,100,000
Percentage increase in probable maximum loss (in hundredths)			0.50%
Increase in reinsurance premiums for probable maximum loss			3,700,000
Percentage increase in reinsurance premiums for probable maximum loss (in hundredths)			8.70%
Trust agreement for loss exposure	4,800,000			4,800,000
2012-2013 Hurricane Season [Member]
Liability for Catastrophe Claims [Line Items]
Aggregate catastrophic loss coverage	328,300,000
Maximum single event coverage	246,500,000
Amount retained per Loss	8,000,000
Coverage afforded by FHCF	144,700,000
Percentage of FHCF coverage to total coverage (in hundredths)	44.10%
Reinsurance premium	9,600,000
Prepaid reinsurance premiums	41,600,000
2011-2012 Hurricane Season [Member]
Liability for Catastrophe Claims [Line Items]
Aggregate catastrophic loss coverage	298,000,000
Maximum single event coverage	226,000,000
Amount retained per Loss	7,000,000
Coverage afforded by FHCF	154,100,000
Percentage of FHCF coverage to total coverage (in hundredths)	51.70%
Reinsurance premium	11,700,000
Prepaid reinsurance premiums	$ 39,700,000