Business Acquisition (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Jul. 02, 2012	Mar. 31, 2012	Dec. 31, 2011
Assets:
Short-term investments, at fair value	$ 81,101	$ 76,502	$ 3,541
Cash and cash equivalents	32,800	20,224	3,176	45,518	43,481
Premiums receivable	28,404	32,559	2,698
Deferred acquisition costs	26,602	26,182	1,186
Goodwill	20,843	20,843	11,526
Intangibles and other assets			9,784
Total assets	1,370,875	1,373,131	31,911	1,298,973
Liabilities:
Unpaid losses and loss adjustment expenses	726,566	725,244	(2,135)
Unearned premiums	148,795	146,096	(2,358)
Ceded premiums payable	17,694	13,386	(1,797)
Funds held	71,815	76,806	(3,504)
Other liabilities	16,660	18,601	(1,057)
Deferred income taxes	7,137	8,767	(3,060)
Total liabilities	$ 1,027,850	$ 1,028,083	$ (13,911)