Losses and Loss Expenses - Reconciliation of the beginning and ending balances of unpaid losses and losee expenses (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2018	Mar. 31, 2017
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses, beginning of period		$ 29,696,779	$ 25,939,571
Unpaid losses and loss expenses recoverable, beginning of period		7,247,723
Net unpaid losses and loss expenses, beginning of period		22,457,333	20,459,271
Increase (decrease) in net losses and loss expenses incurred in respect of losses occurring in:
Current year		1,631,142	1,559,452
Prior year		(9,136)	24,004
Total net incurred losses and loss expenses		1,622,006	1,583,456
Foreign exchange and other		262,571	56,244
Less net losses and loss expenses paid in respect of losses occurring in:
Current year		128,691	106,472
Prior year		1,774,194	1,222,119
Total net paid losses		1,902,885	1,328,591
Net unpaid losses and loss expenses, end of period		22,439,025	20,770,380
Unpaid losses and loss expenses recoverable, end of period		7,271,013
Unpaid losses and loss expenses, end of period		29,701,568	26,445,124
Total P&C
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Unpaid losses and loss expenses recoverable, beginning of period	[1]	7,239,446	5,480,300
Less net losses and loss expenses paid in respect of losses occurring in:
Unpaid losses and loss expenses recoverable, end of period	[1]	$ 7,262,543	$ 5,674,744

[1] P&C business only, net of provision for uncollectible reinsurance.