Schedule VI - Supplementary Information For Property and Casualty Insurance Underwriters (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Net investment income	$ 82,496	$ 80,105	$ 72,990	$ 66,994	$ 67,095	$ 66,083	$ 68,369	$ 65,672	$ 302,585	$ 267,219	$ 294,895
Net Losees and Loss Adjustment Expenses Incurred Related to Current Year									2,246,152	1,943,466	2,082,805
Net Losees and Loss Adjustment Expenses Incurred Related to Prior Years									(326,902)	(264,042)	(221,528)
Consolidated Subsidiaries
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Acquisition Costs, Net	414,525				342,314				414,525	342,314	262,822
Reserves for Losses and Loss Adjustment Expenses	9,036,448				8,824,696				9,036,448	8,824,696	8,933,292
Discount, if any, deducted in Column C	14,732				12,539				14,732	12,539	10,485
Unearned Premiums	2,231,578				1,896,365				2,231,578	1,896,365	1,647,978
Net Premiums Earned									3,593,748	3,145,952	2,935,140
Net Losees and Loss Adjustment Expenses Incurred Related to Current Year									2,246,152	1,943,466	2,082,805
Net Losees and Loss Adjustment Expenses Incurred Related to Prior Years									(326,902)	(264,042)	(221,528)
Amortization of Deferred Acquisition Costs									657,262	564,103	508,884
Net Paid Losses and Loss Adjustment Expenses									1,697,736	1,708,817	1,465,379
Net Premiums Written									$ 3,891,938	$ 3,351,367	$ 3,052,235