Reinsurance - Effects of Reinsurance (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Net premiums written:
Net premiums written	$ 10,242,683	$ 7,951,063	$ 5,944,841
Net premiums earned:
Direct premiums earned	9,142,690	7,717,173	5,173,403
Assumed premiums earned	4,340,132	3,775,337	2,676,765
Ceded premiums earned	(3,704,888)	(3,266,085)	(1,955,098)
Net amount, premiums earned	9,777,934	8,226,425	5,895,070
Property and casualty operations
Net premiums written:
Direct premiums written	9,166,627	7,847,236	5,450,868
Assumed premiums written	4,458,981	2,821,771	2,310,621
Ceded premiums written	(3,394,972)	(2,780,183)	(1,994,280)
Net premiums written	10,230,636	7,888,824	5,767,209
Net premiums earned:
Direct premiums earned	9,142,690	7,717,173	5,173,403
Assumed premiums earned	4,074,817	3,465,421	2,343,329
Ceded premiums earned	(3,451,620)	(3,018,410)	(1,799,294)
Net amount, premiums earned	$ 9,765,887	$ 8,164,184	$ 5,717,438