Losses and Loss Expenses - Information about incurred and paid claims development - Insurance (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
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 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 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 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 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 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 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 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 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 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
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
Insurance Segment | Accident Year 2008
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
Insurance Segment | Accident Year 2009
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
Insurance Segment | Accident Year 2010
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
Insurance Segment | Accident Year 2011
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
Insurance Segment | Accident Year 2012
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
Insurance Segment | Accident Year 2013
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
Insurance Segment | Accident Year 2014
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
Insurance Segment | Accident Year 2015
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
Insurance Segment | Accident Year 2016
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
Insurance Segment | Accident Year 2017
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

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