Reinsurance and Policy Benefits and Losses Claims and Loss Expenses Payable (Policy Benefits and Losses, Claims and Loss Expenses Payable for Property and Casualty Insurance) (Details) - USD ($) $ in Thousands	Mar. 31, 2019		Dec. 31, 2018	Mar. 31, 2018		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance:
Unpaid losses and loss adjustment expense			$ 228,970
Total	$ 1,011,183			$ 1,248,033
Property and Casualty Insurance [Member]
Reinsurance:
Unpaid losses and loss adjustment expense			228,970			$ 233,554	$ 244,400	$ 251,964
Reinsurance losses payable			988			805
Total			$ 229,958			$ 234,359
Operating Segments [Member] | Property and Casualty Insurance [Member]
Reinsurance:
Total	$ 229,958	[1]		$ 234,359	[2]

[1] Balances as of December 31, 2018

[2] Balances as of December 31, 2016