Segment Information (Details) (USD $) In Thousands, unless otherwise specified	3 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
Financial data by segment
Gross written premiums									$ 58,552	$ 57,365	$ 43,404
Net written premiums									255,360	240,618	223,240
Net earned premiums									253,866	232,879	200,733
Fee & other income (loss)									3,365	3,356	4,188
Losses & loss adjustment expenses									171,698	169,367	120,295
Acquisition & other underwriting expenses									106,110	94,310	85,202
Underwriting profit (loss)									(20,577)	(27,442)	(576)
Net investment income									30,198	31,338	32,138
Pre-tax operating income (loss)									9,621	3,896	31,562
Net realized gains									9,693	10,966	2,251
Interest and corporate expenses									5,365	4,132	5,944
Earnings before income taxes									13,949	10,730	27,869
Income tax benefit									841	(3,394)	(2,082)
Net earnings									13,108	14,124	29,951
Less: Net earnings attributable to the non-controlling interest									1,269	3,282	(557)
Net earnings attributable to American Safety Insurance Holdings, Ltd.	644	6,078	1,985	4,401	(4,080)	5,728	4,082	8,394	11,839	10,842	30,508
Loss ratio									67.60%	72.70%	59.90%
Expense ratio									40.50%	39.10%	40.10%
Combined ratio									108.10%	111.80%	100.00%
E & S [Member]
Financial data by segment
Gross written premiums									185,948	155,534	138,166
Net written premiums									148,297	125,107	114,223
Net earned premiums									138,464	119,225	102,907
Fee & other income (loss)									138	(4)	501
Losses & loss adjustment expenses									82,470	60,532	58,119
Acquisition & other underwriting expenses									61,269	50,956	47,621
Underwriting profit (loss)									(5,137)	7,733	(2,332)
Net investment income									17,615	18,572	21,331
Pre-tax operating income (loss)									12,478	26,305	18,999
Loss ratio									59.60%	50.80%	56.50%
Expense ratio									44.10%	42.70%	45.60%
Combined ratio									103.70%	93.50%	102.10%
ART [Member]
Financial data by segment
Gross written premiums									75,590	83,820	88,784
Net written premiums									49,357	57,329	62,366
Net earned premiums									56,466	60,667	53,752
Fee & other income (loss)									3,131	3,203	3,615
Losses & loss adjustment expenses									47,396	57,130	34,663
Acquisition & other underwriting expenses									23,508	24,988	22,068
Underwriting profit (loss)									(11,307)	(18,248)	636
Net investment income									5,173	5,681	4,891
Pre-tax operating income (loss)									(6,134)	(12,567)	5,527
Loss ratio									83.90%	94.20%	64.50%
Expense ratio									36.10%	35.90%	34.30%
Combined ratio									120.00%	130.10%	98.80%
Reinsurance [Member]
Financial data by segment
Gross written premiums									57,706	59,192	50,618
Net written premiums									57,706	58,183	46,658
Net earned premiums									58,936	52,988	44,081
Fee & other income (loss)									17		269
Losses & loss adjustment expenses									42,063	49,705	28,880
Acquisition & other underwriting expenses									17,705	14,785	12,810
Underwriting profit (loss)									(815)	(11,502)	2,660
Net investment income									6,849	6,358	5,231
Pre-tax operating income (loss)									6,034	(5,144)	7,891
Loss ratio									71.40%	93.80%	65.50%
Expense ratio									30.00%	27.90%	28.40%
Combined ratio									101.40%	121.70%	93.90%
Other Run-off [Member]
Financial data by segment
Gross written premiums									0	(1)	(7)
Net written premiums										(1)	(7)
Net earned premiums										(1)	(7)
Fee & other income (loss)									79	157	(197)
Losses & loss adjustment expenses									(231)	2,000	(1,367)
Acquisition & other underwriting expenses									3,628	3,581	2,703
Underwriting profit (loss)									(3,318)	(5,425)	(1,540)
Net investment income									561	727	685
Pre-tax operating income (loss)									$ (2,757)	$ (4,698)	$ (855)