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, 2018		Dec. 31, 2017	Mar. 31, 2017		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance:
Unpaid losses and loss adjustment expense			$ 233,554
Total	$ 1,248,033			$ 1,086,322
Operating Segments [Member] | Property and Casualty Insurance [Member]
Reinsurance:
Unpaid losses and loss adjustment expense			233,554			$ 244,400	$ 251,964	$ 271,609
Reinsurance losses payable			805			580
Total	$ 234,359	[1]	$ 234,359	$ 244,980	[2]	$ 244,980

[1] Balances as of December 31, 2016

[2] Balances as of December 31, 2015