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]
Net deferred acquisition costs	$ 151,546	$ 126,661	$ 108,010
Reserves for losses and loss expenses	5,881,165	5,766,529	5,645,549
Unearned premiums	1,555,313	1,396,256	1,218,021
Net premiums earned	2,182,758	2,005,833	1,748,898
Net investment income	176,874	157,564	167,141
Losses and loss expenses	1,199,190	1,123,242	1,139,264
Amortization of deferred acquisition costs	295,122	252,673	205,722
Other operating costs	365,736	352,285	307,321
Net premiums written	2,322,075	2,120,481	1,837,823
North American Insurance
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	55,756	45,446	38,413
Reserves for losses and loss expenses	3,806,621	3,701,744	3,597,226
Unearned premiums	1,003,151	872,628	763,743
Net premiums earned	1,111,164	1,022,979	892,721
Net investment income	0	0	0
Losses and loss expenses	683,836	651,267	608,762
Amortization of deferred acquisition costs	105,914	94,869	77,701
Other operating costs	219,665	209,042	185,231
Net premiums written	1,230,839	1,082,441	964,199
Global Markets Insurance
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	23,336	9,828	8,326
Reserves for losses and loss expenses	568,126	568,675	567,675
Unearned premiums	223,402	164,792	131,130
Net premiums earned	162,566	126,008	115,660
Net investment income	0	0	0
Losses and loss expenses	61,057	50,395	19,536
Amortization of deferred acquisition costs	18,222	10,051	8,483
Other operating costs	68,106	63,164	48,365
Net premiums written	187,997	145,002	125,577
Reinsurance
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	72,454	71,387	61,271
Reserves for losses and loss expenses	1,506,418	1,496,110	1,480,648
Unearned premiums	328,760	358,836	323,148
Net premiums earned	909,028	856,846	740,517
Net investment income	0	0	0
Losses and loss expenses	454,297	421,580	510,966
Amortization of deferred acquisition costs	170,986	147,753	119,538
Other operating costs	77,965	80,079	73,725
Net premiums written	903,239	893,038	748,047
Corporate
Supplementary Insurance Information, by Segment [Line Items]
Net deferred acquisition costs	0	0	0
Reserves for losses and loss expenses	0	0	0
Unearned premiums	0	0	0
Net premiums earned	0	0	0
Net investment income	176,874	157,564	167,141
Losses and loss expenses	0	0	0
Amortization of deferred acquisition costs	0	0	0
Other operating costs	0	0	0
Net premiums written	$ 0	$ 0	$ 0