Reinsurance - Components of Unpaid Losses and Loss Expenses Recoverable (Details) - USD ($) $ in Thousands		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Analysis of total unpaid losses and loss expenses recoverable by segment:
Unpaid losses and loss expenses recoverable		$ 7,247,723	$ 5,491,297
Reinsurance balances receivable		930,114	577,479
Ceded unearned premiums		2,198,217	1,687,864
Reinsurance Assets		10,376,054	7,756,640
Property and casualty operations
Analysis of total unpaid losses and loss expenses recoverable by segment:
Unpaid losses and loss expenses recoverable	[1]	7,239,446	5,480,300	$ 5,248,905	$ 3,411,528
Corporate and other
Analysis of total unpaid losses and loss expenses recoverable by segment:
Unpaid losses and loss expenses recoverable		$ 8,277	$ 10,997

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