Segment Information (Details 1) (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 about geographic locations
Income tax benefit									$ 841	$ (3,394)	$ (2,082)
Net earnings (loss) 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
Assets	1,373,131				1,286,532				1,373,131	1,286,532	1,221,255
Equity	345,048				334,680				345,048	334,680	318,535
United States [Member]
Financial data about geographic locations
Income tax benefit									841	(3,394)	(2,082)
Net earnings (loss) attributable to American Safety Insurance Holdings, Ltd.									60	(7,029)	918
Assets	719,235				671,619				719,235	671,619	652,109
Equity	140,011				106,689				140,011	106,689	99,373
Bermuda [Member]
Financial data about geographic locations
Net earnings (loss) attributable to American Safety Insurance Holdings, Ltd.									11,779	17,871	29,590
Assets	653,896				614,913				653,896	614,913	569,146
Equity	$ 205,037				$ 227,991				$ 205,037	$ 227,991	$ 219,162