Schedule III - Supplementary Insurance Information (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	$ 414,525		$ 342,314		$ 262,822
Reserves for Losses and Loss Adjustment Expenses	9,036,448		8,824,696		8,933,292
Unearned Premiums	2,231,578		1,896,365		1,647,978
Net Premiums Earned	3,593,748		3,145,952		2,935,140
Net Losses and Loss Adjustment Expenses Incurred	1,919,250		1,679,424		1,861,277
Amortization of Deferred Acquisition Costs	657,262		564,103		508,884
Other Operating Expenses	556,280	[1]	458,327	[1]	430,035	[1]
Net Premiums Written	3,891,938		3,351,367		3,052,235
Insurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	178,545		158,121		141,962
Reserves for Losses and Loss Adjustment Expenses	6,161,500		6,137,121		6,149,247
Unearned Premiums	1,237,099		1,192,188		1,077,211
Net Premiums Earned	2,017,370		1,876,014		1,800,343
Net Losses and Loss Adjustment Expenses Incurred	1,260,953		1,188,445		1,283,841
Amortization of Deferred Acquisition Costs	316,308		311,904		298,983
Other Operating Expenses	335,157	[1]	315,387	[1]	307,489	[1]
Net Premiums Written	2,146,654		1,948,796		1,825,334
Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	148,130		167,642		106,677
Reserves for Losses and Loss Adjustment Expenses	2,690,070		2,680,288		2,781,424
Unearned Premiums	666,233		612,725		517,619
Net Premiums Earned	1,279,328		1,218,672		1,118,127
Net Losses and Loss Adjustment Expenses Incurred	532,450		486,236		574,821
Amortization of Deferred Acquisition Costs	261,438		234,373		204,903
Other Operating Expenses	147,964	[1]	134,563	[1]	118,245	[1]
Net Premiums Written	1,265,991		1,313,001		1,158,790
Mortgage
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	38,321		16,551		14,183
Reserves for Losses and Loss Adjustment Expenses	118,550		7,287		2,621
Unearned Premiums	148,232		91,452		53,148
Net Premiums Earned	193,573		51,266		16,670
Net Losses and Loss Adjustment Expenses Incurred	55,674		4,743		2,615
Amortization of Deferred Acquisition Costs	49,400		17,826		4,998
Other Operating Expenses	66,891	[1]	8,377	[1]	4,301	[1]
Net Premiums Written	204,837		89,570		68,111
Other
Supplementary Insurance Information, by Segment [Line Items]
Deferred Acquisition Costs, Net	49,529
Reserves for Losses and Loss Adjustment Expenses	66,328
Unearned Premiums	180,014
Net Premiums Earned	103,477
Net Losses and Loss Adjustment Expenses Incurred	70,173
Amortization of Deferred Acquisition Costs	30,116
Other Operating Expenses	6,268	[1]
Net Premiums Written	$ 274,456

[1] Certain other operating expenses relate to the Company’s corporate segment (non-underwriting). Such amounts are not reflected in the table above. See Note 5.