Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Life reinsurance in force:
Gross amount, life reinsurance in force	[1]	$ 0				$ 0				$ 0	$ 0	$ 0
Ceded to other companies, life reinsurance in force	[1]	51,037,696				51,176,725				51,037,696	51,176,725	62,290,992
Assumed from other companies, life reinsurance in force	[1]	51,847,908				51,971,475				51,847,908	51,971,475	63,158,304
Net amount, life reinsurance in force	[1]	810,212				794,750				810,212	794,750	867,312
Total premiums earned:
Direct amount, premiums earned										9,078,575	9,142,690	7,717,173
Ceded to other companies, premiums earned										3,880,150	3,704,888	3,266,085
Assumed from other companies, premiums earned										5,138,187	4,340,132	3,775,337
Net amount, premiums earned		$ 2,676,002	$ 2,620,902	$ 2,516,917	$ 2,522,791	$ 2,457,075	$ 2,434,037	$ 2,532,212	$ 2,354,610	$ 10,336,612	$ 9,777,934	$ 8,226,425
Percentage of amount assumed to net										49.70%	44.40%	45.90%
Property and casualty operations
Total premiums earned:
Direct amount, premiums earned										$ 9,078,575	$ 9,142,690	$ 7,717,173
Ceded to other companies, premiums earned										3,658,206	3,451,620	3,018,410
Assumed from other companies, premiums earned										4,903,909	4,074,817	3,465,421
Net amount, premiums earned										$ 10,324,278	$ 9,765,887	$ 8,164,184
Percentage of amount assumed to net										47.50%	41.70%	42.40%
Life operations
Total premiums earned:
Direct amount, premiums earned										$ 0	$ 0	$ 0
Ceded to other companies, premiums earned										221,944	253,268	247,675
Assumed from other companies, premiums earned										234,278	265,315	309,916
Net amount, premiums earned										$ 12,334	$ 12,047	$ 62,241

[1] Represents the sum face value outstanding of the in force life reinsurance policies.