Reinsurance - Components of Unpaid Losses and Loss Expenses Recoverable (Details) - USD ($) $ in Thousands		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Analysis of total unpaid losses and loss expenses recoverable by segment:
Unpaid losses and loss expenses recoverable		$ 5,491,297	$ 5,262,706
Property and casualty operations
Analysis of total unpaid losses and loss expenses recoverable by segment:
Unpaid losses and loss expenses recoverable	[1]	5,480,300	5,248,905	$ 3,411,528	$ 3,414,735
Corporate and other
Analysis of total unpaid losses and loss expenses recoverable by segment:
Unpaid losses and loss expenses recoverable		$ 10,997	$ 13,801

[1] Property and Casualty business only.