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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Premiums written:
Direct	$ 714,234	$ 683,820
Assumed	6,271	5,691
Ceded	(95,945)	(90,807)
Net	624,560	598,704
Premiums earned:
Direct	683,733	646,728
Assumed	6,124	5,779
Ceded	(98,029)	(91,653)
Net	591,828	560,854
Loss and loss expenses incurred:
Direct	420,916	342,122
Assumed	8,004	4,437
Ceded	(43,979)	(29,087)
Net	384,941	317,472
National Flood Insurance Program [Member]
Premiums written:
Ceded	(56,669)	(56,334)
Premiums earned:
Ceded	(58,991)	(57,277)
Loss and loss expenses incurred:
Ceded	$ (15,719)	$ (6,541)

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