Unaudited Condensed Quarterly Financial Information (Details) (USD $) In Thousands, except Per Share data, 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
Quarterly Financial Information Disclosure [Abstract]
Net premiums written	$ 895,481	$ 959,539	$ 971,928	$ 1,064,990	$ 748,921	$ 839,135	$ 810,535	$ 952,776	$ 3,891,938	$ 3,351,367	$ 3,052,235
Net premiums earned	923,165	903,651	907,152	859,780	839,366	795,000	758,816	752,770	3,593,748	3,145,952	2,935,140
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 realized gains	10,561	18,515	54,144	19,697	9,048	(6,022)	12,652	58,340	102,917	74,018	194,228
Net impairment losses recognized in earnings	(3,837)	(8,593)	(14,749)	(2,971)	(88)	(728)	(724)	(2,246)	(30,150)	(3,786)	(11,388)
Underwriting Income Loss Segment	113,464	101,087	124,091	132,456	128,318	110,992	96,029	116,398	471,098	451,737	143,034
Net income	207,134	223,264	211,717	179,145	161,490	114,825	176,940	256,476	821,260	709,731	593,397
Dividends declared on preferred shares	(5,485)	(5,484)	(5,485)	(5,484)	(5,485)	(5,484)	(5,485)	(5,484)	(21,938)	(21,938)	(25,079)
Net income available to common shareholders	209,679	223,191	202,531	177,016	156,005	109,341	171,455	250,992	812,417	687,793	557,706
Loss on repurchase of preferred shares									$ 0	$ 0	$ 10,612
Basic (per share)	$ 1.65	$ 1.69	$ 1.53	$ 1.34	$ 1.19	$ 0.83	$ 1.31	$ 1.92	$ 6.21	$ 5.24	$ 4.15
Diluted (per share)	$ 1.60	$ 1.64	$ 1.48	$ 1.30	$ 1.14	$ 0.80	$ 1.26	$ 1.85	$ 6.02	$ 5.07	$ 4.03