Segments (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Information by business segment
Net earned premium	$ 562,612	$ 561,186
Other revenue	86,318	73,180
Total revenue	648,930	634,366
Loss and LAE	321,844	332,697
Other expense	172,114	150,273
Segment expense	493,958	482,970
Segment pretax earnings (loss)	154,972	151,396
U.S. Property & Casualty [Member]
Information by business segment
Net earned premium	97,052	93,531
Other revenue	5,876	7,184
Total revenue	102,928	100,715
Loss and LAE	46,655	52,156
Other expense	29,093	27,305
Segment expense	75,748	79,461
Segment pretax earnings (loss)	27,180	21,254
Professional Liability [Member]
Information by business segment
Net earned premium	85,450	92,779
Other revenue	275	(414)
Total revenue	85,725	92,365
Loss and LAE	51,132	56,386
Other expense	17,125	17,748
Segment expense	68,257	74,134
Segment pretax earnings (loss)	17,468	18,231
Accident & Health [Member]
Information by business segment
Net earned premium	232,143	217,125
Other revenue	1,640	1,190
Total revenue	233,783	218,315
Loss and LAE	168,999	160,427
Other expense	36,379	31,126
Segment expense	205,378	191,553
Segment pretax earnings (loss)	28,405	26,762
U.S. Surety & Credit [Member]
Information by business segment
Net earned premium	46,943	47,177
Other revenue	288	237
Total revenue	47,231	47,414
Loss and LAE	13,696	13,214
Other expense	25,944	26,279
Segment expense	39,640	39,493
Segment pretax earnings (loss)	7,591	7,921
International [Member]
Information by business segment
Net earned premium	100,733	105,142
Other revenue	968	778
Total revenue	101,701	105,920
Loss and LAE	41,615	45,919
Other expense	38,677	35,709
Segment expense	80,292	81,628
Segment pretax earnings (loss)	21,409	24,292
Investing [Member]
Information by business segment
Net earned premium	0	0
Other revenue	77,052	64,335
Total revenue	77,052	64,335
Loss and LAE	0	0
Other expense	0	0
Segment expense	0	0
Segment pretax earnings (loss)	77,052	64,335
Corporate & Other [Member]
Information by business segment
Net earned premium	291	5,432
Other revenue	219	(130)
Total revenue	510	5,302
Loss and LAE	(253)	4,595
Other expense	24,896	12,106
Segment expense	24,643	16,701
Segment pretax earnings (loss)	$ (24,133)	$ (11,399)