Schedule VI - Supplementary Information Concerning Property/Casualty Insurance Operations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Losses and loss expenses incurred related to - current year	$ 6,374,377	$ 5,072,830	$ 3,513,465
Losses and loss expenses incurred related to - prior year	(301,542)	(306,630)	(255,072)
P&C Operations
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred acquisition costs	947,702	893,111	357,067
Reserves for losses and loss expenses	25,939,571	25,439,744	19,353,241
Reserves for unearned premiums	7,292,593	7,043,383	3,973,159
Net earned premiums	9,765,887	8,164,184	5,717,438
Net investment income	640,656	644,312	642,492
Losses and loss expenses incurred related to - current year	6,374,377	5,072,830	3,513,465
Losses and loss expenses incurred related to - prior year	(301,542)	(306,630)	(255,072)
Net paid losses and loss expenses	5,334,520	5,374,652	3,821,335
Amortization of deferred acquisition costs	1,614,896	1,306,654	724,003
Net premiums written	$ 10,230,636	$ 7,888,824	$ 5,767,209