Losses and Loss Expenses - Information about incurred and paid claims development - Reinsurance (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		$ 53,162,870
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		35,612,206
All outstanding liabilities prior to 2007, net of reinsurance		2,501,779
Liabilities for unpaid losses and loss expenses, net of reinsurance		20,052,443
Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		15,130,780
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		9,853,346
All outstanding liabilities prior to 2007, net of reinsurance		1,431,871
Liabilities for unpaid losses and loss expenses, net of reinsurance		6,709,305
Accident Year 2007
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,390,981	$ 4,432,150	$ 4,519,599	$ 4,541,978	$ 4,569,019	$ 4,689,256	$ 4,684,540	$ 4,718,961	$ 4,806,850	$ 4,632,657
IBNR (1)	[1]	246,827
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,894,280	3,822,207	3,744,872	3,612,934	3,397,370	3,126,834	2,587,918	2,055,288	1,289,961	521,083
Accident Year 2007 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,144,626	1,157,375	1,201,176	1,224,302	1,239,171	1,246,925	1,252,940	1,292,265	1,315,895	1,373,989
IBNR (1)	[1]	88,476
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		951,136	925,230	922,262	897,073	848,114	793,787	703,600	592,431	359,533	$ 153,912
Accident Year 2008
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,322,204	5,357,648	5,417,072	5,449,364	5,479,907	5,524,325	5,503,422	5,575,015	5,504,796
IBNR (1)	[1]	307,738
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,700,567	4,582,409	4,450,875	4,240,640	3,947,203	3,446,415	2,857,474	1,994,235	788,760
Accident Year 2008 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,535,533	1,535,515	1,559,314	1,559,826	1,561,457	1,550,254	1,567,923	1,605,756	1,613,114
IBNR (1)	[1]	121,015
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,285,355	1,244,802	1,216,006	1,170,828	1,104,015	1,019,382	883,897	654,658	$ 327,154
Accident Year 2009
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,392,904	4,422,167	4,550,542	4,550,362	4,556,902	4,542,408	4,627,569	4,622,547
IBNR (1)	[1]	317,583
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,663,224	3,551,474	3,366,917	3,117,922	2,655,163	2,192,791	1,530,295	638,179
Accident Year 2009 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,046,087	1,052,553	1,049,967	1,067,335	1,076,187	1,084,088	1,128,445	1,150,325
IBNR (1)	[1]	127,390
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		797,666	764,307	732,134	685,901	611,967	526,118	389,499	$ 176,089
Accident Year 2010
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,981,939	4,957,474	4,974,782	5,026,101	5,002,061	5,111,634	4,983,044
IBNR (1)	[1]	464,306
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,084,244	3,851,827	3,521,163	3,144,278	2,645,868	1,855,407	840,030
Accident Year 2010 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,284,178	1,295,555	1,286,737	1,290,292	1,302,827	1,302,532	1,280,586
IBNR (1)	[1]	150,491
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,011,698	936,971	836,148	774,216	667,841	477,894	$ 232,372
Accident Year 2011
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,083,796	6,116,046	5,980,635	6,111,779	6,187,474	6,236,775
IBNR (1)	[1]	558,008
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		5,021,000	4,737,827	4,242,053	3,664,970	2,872,319	1,246,596
Accident Year 2011 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		2,082,525	2,089,535	2,059,076	2,102,613	2,201,054	2,222,386
IBNR (1)	[1]	178,659
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,730,787	1,675,428	1,588,434	1,448,330	1,255,164	$ 596,906
Accident Year 2012
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,663,926	5,754,387	5,640,576	5,660,616	5,697,958
IBNR (1)	[1]	715,827
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,199,766	3,802,042	3,149,842	2,256,973	952,158
Accident Year 2012 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,812,667	1,832,972	1,840,530	1,860,740	1,827,087
IBNR (1)	[1]	247,825
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,313,092	1,223,403	1,070,442	785,212	$ 318,115
Accident Year 2013
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,426,144	5,523,695	5,554,791	5,558,316
IBNR (1)	[1]	1,082,491
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,614,076	3,049,907	2,194,912	976,023
Accident Year 2013 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,502,096	1,544,739	1,551,432	1,615,956
IBNR (1)	[1]	279,880
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,016,654	908,575	685,680	$ 318,270
Accident Year 2014
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,397,264	5,369,393	5,183,056
IBNR (1)	[1]	1,396,883
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,090,343	2,250,446	957,525
Accident Year 2014 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,418,828	1,452,029	1,434,902
IBNR (1)	[1]	344,569
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		826,700	653,323	$ 287,780
Accident Year 2015
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,498,206	5,354,494
IBNR (1)	[1]	1,962,006
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		2,152,569	970,277
Accident Year 2015 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,459,003	1,504,763
IBNR (1)	[1]	543,530
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		589,550	$ 279,024
Accident Year 2016
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,005,506
IBNR (1)	[1]	3,425,518
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,192,137
Accident Year 2016 | Reinsurance Segment
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,845,237
IBNR (1)	[1]	1,067,346
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 330,708

[1] Total IBNR Liabilities Plus Expected Development on Reported Claims