Reinsurance - Effects of 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
Net premiums written:
Net premiums written									$ 10,680,758	$ 10,242,683	$ 7,951,063
Net premiums earned:
Direct premiums earned									9,078,575	9,142,690	7,717,173
Assumed premiums earned									5,138,187	4,340,132	3,775,337
Ceded premiums earned									(3,880,150)	(3,704,888)	(3,266,085)
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
Property and casualty operations
Net premiums written:
Direct premiums written									8,965,259	9,166,627	7,847,236
Assumed premiums written									5,787,313	4,458,981	2,821,771
Ceded premiums written									(4,084,148)	(3,394,972)	(2,780,183)
Net premiums written									10,668,424	10,230,636	7,888,824
Net premiums earned:
Direct premiums earned									9,078,575	9,142,690	7,717,173
Assumed premiums earned									4,903,909	4,074,817	3,465,421
Ceded premiums earned									(3,658,206)	(3,451,620)	(3,018,410)
Net amount, premiums earned									$ 10,324,278	$ 9,765,887	$ 8,164,184