Reserve for Loss and Loss Settlement Expenses (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Liability for Unpaid Claims and Claims Adjustment Expense [Roll Forward]
Gross liability for losses and loss settlement expense at beginning of year	$ 945,051	$ 603,090	$ 606,045
Ceded loss and loss settlement expense	(120,359)	(39,000)	(33,754)
Net liability for losses and loss settlement expenses at beginning of year	824,692	564,090	572,291
Reserves acquired in Mercer Insurance Group acquisition, net		252,598
Beginning balance, as adjusted		816,688
Losses and loss settlement expenses incurred for claims occurring during current year	512,564	468,926	335,315
Losses and loss settlement expenses incurred for claims occurring during prior years	(73,427)	(61,095)	(45,878)
Losses and loss settlement expenses incurred, Total	439,137	407,831	289,437
Losses and loss settlement expense payments for claims occurring during current year	201,632	253,175	132,592
Losses and loss settlement expense payments for claims occurring during prior years	194,156	146,653	165,046
Losses and loss settlement expense payments, Total	395,788	399,828	297,638
Net liability for losses and loss settlement expenses at end of year	868,041	824,692	564,090
Ceded loss and loss settlement expense	103,870	120,359	39,000
Gross liability for losses and loss settlement expense at end of year	$ 971,911	$ 945,051	$ 603,090