Losses and Loss Expenses - Information about incurred and paid claims development - Insurance - Casualty (Details) $ in Thousands		Dec. 31, 2016 USD ($) Claim	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)	Dec. 31, 2009 USD ($)	Dec. 31, 2008 USD ($)	Dec. 31, 2007 USD ($)
Claims Development [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
Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		38,032,090
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		25,758,860
All outstanding liabilities prior to 2007, net of reinsurance		1,069,908
Liabilities for unpaid losses and loss expenses, net of reinsurance		13,343,138
Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		12,501,951
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		6,913,174
All outstanding liabilities prior to 2007, net of reinsurance		765,300
Liabilities for unpaid losses and loss expenses, net of reinsurance		6,354,077
Accident Year 2007
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,246,355	3,274,775	3,318,423	3,317,676	3,329,848	3,442,331	3,431,600	3,426,696	3,490,955	3,258,668
IBNR (1)	[1]	$ 158,351
Number of Claims | Claim		45,542
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,943,144	2,896,977	2,822,610	2,715,861	2,549,256	2,333,047	1,884,318	1,462,857	930,428	367,171
Accident Year 2007 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,162,152	1,171,058	1,179,115	1,163,024	1,159,030	1,175,846	1,166,693	1,087,608	1,112,540	1,082,033
IBNR (1)	[1]	$ 85,633
Number of Claims | Claim		17,237
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,011,066	993,114	961,739	917,798	855,446	751,437	470,594	315,942	142,855	$ 22,680
Accident Year 2008
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,786,671	3,822,133	3,857,758	3,889,538	3,918,450	3,974,071	3,935,499	3,969,259	3,891,682
IBNR (1)	[1]	$ 186,723
Number of Claims | Claim		54,952
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,415,212	3,337,607	3,234,869	3,069,812	2,843,188	2,427,033	1,973,577	1,339,577	461,606
Accident Year 2008 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,078,619	1,085,898	1,102,780	1,105,909	1,093,984	1,106,258	1,112,589	1,112,464	1,140,247
IBNR (1)	[1]	$ 111,716
Number of Claims | Claim		18,669
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 898,297	886,979	843,167	771,029	673,130	557,222	384,593	175,724	$ 46,589
Accident Year 2009
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,346,817	3,369,614	3,500,575	3,483,027	3,480,715	3,458,320	3,499,124	3,472,222
IBNR (1)	[1]	$ 190,193
Number of Claims | Claim		54,896
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,865,558	2,787,167	2,634,783	2,432,021	2,043,196	1,666,673	1,140,796	462,090
Accident Year 2009 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,041,605	1,048,630	1,089,350	1,064,751	1,083,830	1,089,006	1,101,930	1,116,831
IBNR (1)	[1]	$ 104,475
Number of Claims | Claim		20,836
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 818,732	771,855	727,782	638,617	535,197	414,586	237,264	$ 43,912
Accident Year 2010
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,697,761	3,661,919	3,688,045	3,735,809	3,699,234	3,809,102	3,702,458
IBNR (1)	[1]	$ 313,815
Number of Claims | Claim		61,416
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,072,546	2,914,856	2,685,015	2,370,062	1,978,027	1,377,513	607,658
Accident Year 2010 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,230,374	1,233,729	1,276,555	1,253,471	1,258,380	1,250,384	1,112,521
IBNR (1)	[1]	$ 193,273
Number of Claims | Claim		23,029
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 937,451	862,584	759,673	649,167	516,383	292,403	$ 101,673
Accident Year 2011
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,001,271	4,026,511	3,921,559	4,009,166	3,986,420	4,014,389
IBNR (1)	[1]	$ 379,349
Number of Claims | Claim		65,437
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,290,213	3,062,399	2,653,619	2,216,640	1,617,155	649,690
Accident Year 2011 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,138,849	1,160,467	1,134,783	1,160,676	1,139,829	1,092,125
IBNR (1)	[1]	$ 175,768
Number of Claims | Claim		24,942
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 818,838	748,537	594,047	418,204	258,861	$ 115,807
Accident Year 2012
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,851,259	3,921,415	3,800,046	3,799,876	3,870,871
IBNR (1)	[1]	$ 468,002
Number of Claims | Claim		70,919
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,886,674	2,578,639	2,079,400	1,471,761	634,043
Accident Year 2012 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,217,096	1,258,868	1,235,541	1,212,625	1,195,390
IBNR (1)	[1]	$ 229,162
Number of Claims | Claim		26,613
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 775,521	644,936	471,949	297,186	$ 117,735
Accident Year 2013
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,924,048	3,978,956	4,003,359	3,942,360
IBNR (1)	[1]	$ 802,611
Number of Claims | Claim		78,018
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,597,422	2,141,332	1,509,232	657,753
Accident Year 2013 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,375,254	1,403,279	1,344,357	1,295,006
IBNR (1)	[1]	$ 407,549
Number of Claims | Claim		29,179
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 710,905	533,465	307,950	$ 124,226
Accident Year 2014
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,978,436	3,917,364	3,748,154
IBNR (1)	[1]	$ 1,052,314
Number of Claims | Claim		87,491
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,263,643	1,597,123	669,745
Accident Year 2014 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,463,809	1,417,460	1,351,268
IBNR (1)	[1]	$ 607,888
Number of Claims | Claim		30,254
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 555,967	335,216	$ 133,592
Accident Year 2015
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,039,203	3,849,731
IBNR (1)	[1]	$ 1,418,476
Number of Claims | Claim		94,229
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,563,019	691,253
Accident Year 2015 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,399,039	1,335,856
IBNR (1)	[1]	$ 722,174
Number of Claims | Claim		31,862
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 282,670	$ 109,217
Accident Year 2016
Claims Development [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 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,160,269
IBNR (1)	[1]	$ 2,358,172
Number of Claims | Claim		65,684
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 861,429
Accident Year 2016 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,395,154
IBNR (1)	[1]	$ 1,088,868
Number of Claims | Claim		18,283
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 103,727

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