Losses and Loss Expenses - Information about incurred and paid claims development - Insurance - Professional (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
Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		9,978,550
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		6,082,903
All outstanding liabilities prior to 2008, net of reinsurance		160,193
Liabilities for unpaid losses and loss expenses, net of reinsurance		4,055,840
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,131,120	1,140,790	1,162,130	1,194,919	1,198,484	1,218,280	1,239,730	1,147,534	1,101,965	1,047,124
IBNR (1)	[1]	$ 26,453
Number of Claims | Claim		4,165
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 996,837	986,705	924,630	865,795	793,754	706,956	522,556	350,967	186,817	$ 18,371
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,199,359	1,192,299	1,196,459	1,229,584	1,219,714	1,180,339	1,112,512	1,050,410	981,970
IBNR (1)	[1]	$ 68,143
Number of Claims | Claim		4,851
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 982,106	965,159	937,925	852,865	758,394	516,995	341,997	162,488	$ 22,596
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,053,011	1,081,736	1,035,210	1,013,567	1,053,660	975,366	1,003,305	1,006,646
IBNR (1)	[1]	$ 56,712
Number of Claims | Claim		4,517
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 851,235	816,307	746,509	632,809	487,964	330,563	157,847	$ 22,132
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,130,481	1,133,524	1,123,136	1,050,516	1,035,722	978,863	987,023
IBNR (1)	[1]	$ 141,147
Number of Claims | Claim		4,203
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 890,169	827,268	691,769	486,621	319,404	146,330	$ 29,116
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		1,155,566	1,142,555	1,148,277	1,033,040	997,484	995,783
IBNR (1)	[1]	$ 174,871
Number of Claims | Claim		4,295
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 839,544	723,313	599,630	382,218	197,103	$ 44,763
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		863,145	938,328	960,557	982,485	984,798
IBNR (1)	[1]	$ 159,645
Number of Claims | Claim		4,503
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 587,897	465,463	309,475	154,971	$ 36,847
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		883,975	857,762	815,411	762,491
IBNR (1)	[1]	$ 272,364
Number of Claims | Claim		4,602
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 453,178	315,969	140,718	$ 17,185
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		935,466	816,200	772,779
IBNR (1)	[1]	$ 372,222
Number of Claims | Claim		5,120
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 319,050	140,307	$ 22,667
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		772,377	750,041
IBNR (1)	[1]	$ 477,454
Number of Claims | Claim		4,490
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 141,813	$ 23,688
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 | Professional lines | Insurance Segment
Claims Development [Line Items]
Incurred Losses and Allocated Loss Expenses, Net of Reinsurance		854,050
IBNR (1)	[1]	$ 727,596
Number of Claims | Claim		2,530
Cumulative Paid Losses and Allocated Loss Expenses, Net of Reinsurance		$ 21,074

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