Reinsurance (Reinsurance Financial Information) (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Disclosures [Abstract]
Written premiums, direct	$ 1,400,412,000	$ 1,201,019,000
Written premiums, assumed	231,209,000	200,507,000
Written premiums, ceded	(254,655,000)	(198,000,000)
Total net premiums written	1,376,966,000	1,203,526,000
Earned premiums, direct	1,240,562,000	1,103,956,000
Earned premiums, assumed	214,016,000	179,518,000
Earned premiums, ceded	(222,459,000)	(183,823,000)
Net premiums earned	1,232,119,000	1,099,651,000	4,673,516,000	4,160,867,000	3,835,582,000
Ceded losses incurred	110,551,000	85,277,000
Uncollectible reinsurance	$ 2,000,000