Losses and Loss Expenses - Information about incurred and paid claims development - Insurance - Casualty (Details) $ in Thousands		Dec. 31, 2017 USD ($) Claim	Dec. 31, 2016 USD ($)	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 ($)	Dec. 31, 2008 USD ($)
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		$ 58,562,228
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		38,846,447
All outstanding liabilities prior to 2008, net of reinsurance		2,451,126
Liabilities for unpaid losses and loss expenses, net of reinsurance		22,166,907
Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		41,174,961
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		27,809,329
All outstanding liabilities prior to 2008, net of reinsurance		988,847
Liabilities for unpaid losses and loss expenses, net of reinsurance		14,354,479
Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		13,256,771
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		7,413,269
All outstanding liabilities prior to 2008, net of reinsurance		682,630
Liabilities for unpaid losses and loss expenses, net of reinsurance		6,526,132
Accident Year 2008
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,410,125	$ 5,471,441	$ 5,510,523	$ 5,572,963	$ 5,603,554	$ 5,631,593	$ 5,674,761	$ 5,651,657	$ 5,735,428	$ 5,644,208
IBNR (1)	[1]	213,925
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,897,567	4,827,462	4,704,623	4,568,478	4,352,467	4,049,900	3,539,510	2,934,270	2,051,520	810,334
Accident Year 2008 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,859,215	3,898,706	3,934,337	3,970,698	4,000,716	4,027,990	4,083,129	4,042,042	4,082,887	3,980,331
IBNR (1)	[1]	$ 125,683
Number of Claims | Claim		48,668
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,561,228	3,512,792	3,431,608	3,325,230	3,156,209	2,922,681	2,498,765	2,032,058	1,383,392	476,792
Accident Year 2008 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,104,152	1,124,751	1,131,461	1,152,489	1,151,725	1,139,479	1,155,409	1,168,244	1,163,391	1,190,556
IBNR (1)	[1]	$ 88,733
Number of Claims | Claim		17,401
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 975,050	937,613	925,434	877,315	801,190	699,167	580,050	402,374	186,940	$ 48,908
Accident Year 2009
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,542,173	4,525,234	4,555,586	4,685,356	4,680,173	4,689,064	4,668,243	4,755,362	4,763,117
IBNR (1)	[1]	293,117
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,844,074	3,767,363	3,650,893	3,460,344	3,204,371	2,731,005	2,254,270	1,573,893	657,811
Accident Year 2009 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,467,258	3,447,870	3,469,966	3,601,916	3,580,582	3,579,030	3,551,370	3,589,904	3,566,518
IBNR (1)	[1]	$ 179,179
Number of Claims | Claim		48,912
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,002,238	2,946,407	2,864,085	2,706,905	2,498,379	2,101,695	1,712,626	1,172,186	477,677
Accident Year 2009 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,089,588	1,083,048	1,089,670	1,129,485	1,109,160	1,128,264	1,133,848	1,146,022	1,160,114
IBNR (1)	[1]	$ 103,833
Number of Claims | Claim		18,965
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 876,503	849,231	800,388	757,980	664,934	555,172	428,450	242,401	$ 44,897
Accident Year 2010
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,053,432	5,138,545	5,109,862	5,127,836	5,178,036	5,158,742	5,272,265	5,141,028
IBNR (1)	[1]	300,001
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,358,655	4,212,573	3,971,942	3,634,321	3,252,357	2,740,308	1,926,916	873,754
Accident Year 2010 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,737,724	3,807,684	3,769,113	3,794,776	3,842,324	3,805,979	3,920,528	3,806,180
IBNR (1)	[1]	$ 190,030
Number of Claims | Claim		55,114
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,273,536	3,163,363	3,001,709	2,764,877	2,444,498	2,041,175	1,423,331	627,377
Accident Year 2010 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,242,812	1,281,469	1,286,535	1,329,461	1,304,841	1,311,718	1,300,366	1,154,053
IBNR (1)	[1]	$ 113,201
Number of Claims | Claim		21,202
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,059,853	997,802	921,367	795,381	674,950	533,756	302,471	$ 104,732
Accident Year 2011
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,226,953	6,276,173	6,311,990	6,175,271	6,306,319	6,392,279	6,430,567
IBNR (1)	[1]	433,196
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		5,375,172	5,175,876	4,883,707	4,373,375	3,778,977	2,963,941	1,285,733
Accident Year 2011 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,083,928	4,127,808	4,154,898	4,049,553	4,139,325	4,120,075	4,140,334
IBNR (1)	[1]	$ 292,990
Number of Claims | Claim		59,373
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,535,878	3,394,821	3,159,121	2,739,159	2,289,261	1,675,220	674,219
Accident Year 2011 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,159,113	1,184,083	1,208,536	1,180,848	1,213,931	1,189,954	1,138,416
IBNR (1)	[1]	$ 127,326
Number of Claims | Claim		23,227
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 913,640	851,135	776,181	614,962	433,116	268,749	$ 119,347
Accident Year 2012
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,827,240	5,837,626	5,930,172	5,808,941	5,836,701	5,869,907
IBNR (1)	[1]	566,059
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,665,888	4,312,155	3,903,535	3,232,741	2,317,760	979,204
Accident Year 2012 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,973,579	3,984,388	4,055,833	3,923,936	3,931,116	3,991,564
IBNR (1)	[1]	$ 364,324
Number of Claims | Claim		64,064
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,228,221	2,976,781	2,660,153	2,145,780	1,520,685	658,877
Accident Year 2012 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,265,720	1,272,795	1,314,186	1,289,302	1,266,356	1,249,618
IBNR (1)	[1]	$ 174,838
Number of Claims | Claim		24,231
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 915,108	808,305	672,066	490,655	308,891	$ 122,267
Accident Year 2013
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,526,922	5,632,776	5,739,200	5,777,217	5,767,068
IBNR (1)	[1]	652,927
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		4,207,557	3,755,115	3,171,205	2,285,432	1,009,683
Accident Year 2013 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		3,982,810	4,064,032	4,125,518	4,148,375	4,072,919
IBNR (1)	[1]	$ 449,668
Number of Claims | Claim		70,379
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 3,039,989	2,691,262	2,220,578	1,567,382	683,757
Accident Year 2013 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,439,636	1,433,238	1,465,886	1,405,834	1,351,663
IBNR (1)	[1]	$ 255,701
Number of Claims | Claim		25,994
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 908,948	741,113	555,409	320,398	$ 130,466
Accident Year 2014
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,574,378	5,580,769	5,549,389	5,368,693
IBNR (1)	[1]	987,715
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,765,670	3,188,259	2,322,853	985,112
Accident Year 2014 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,126,635	4,111,762	4,049,578	3,873,356
IBNR (1)	[1]	$ 759,437
Number of Claims | Claim		80,946
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,771,499	2,338,259	1,650,664	692,365
Accident Year 2014 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,523,376	1,525,064	1,478,925	1,409,229
IBNR (1)	[1]	$ 446,153
Number of Claims | Claim		27,327
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 778,106	579,352	350,433	$ 139,761
Accident Year 2015
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,864,953	5,678,720	5,540,823
IBNR (1)	[1]	1,405,335
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		3,277,189	2,217,824	997,289
Accident Year 2015 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,348,859	4,168,619	3,975,009
IBNR (1)	[1]	$ 1,058,392
Number of Claims | Claim		94,968
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 2,396,659	1,613,855	715,111
Accident Year 2015 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,519,157	1,450,524	1,383,556
IBNR (1)	[1]	$ 571,222
Number of Claims | Claim		29,395
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 582,698	294,471	$ 114,235
Accident Year 2016
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		6,427,610	6,182,999
IBNR (1)	[1]	2,013,460
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		2,769,701	1,233,032
Accident Year 2016 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		4,526,453	4,294,206
IBNR (1)	[1]	$ 1,459,995
Number of Claims | Claim		102,466
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,923,192	889,975
Accident Year 2016 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,465,968	1,447,018
IBNR (1)	[1]	$ 809,907
Number of Claims | Claim		29,188
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 281,722	$ 107,946
Accident Year 2017
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		8,108,442
IBNR (1)	[1]	4,386,436
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		1,684,974
Accident Year 2017 | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		5,068,500
IBNR (1)	[1]	$ 2,435,748
Number of Claims | Claim		73,518
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 1,076,889
Accident Year 2017 | Casualty and other | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,447,249
IBNR (1)	[1]	$ 1,028,669
Number of Claims | Claim		20,498
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 121,641

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