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Reinsurance (List of Direct, Assumed, and Ceded Reinsurance Amounts) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Premiums written:
Direct	$ 752,834	$ 706,918	$ 2,220,431	$ 2,097,146
Assumed	7,084	8,506	19,891	20,685
Ceded	(108,250)	(111,147)	(308,846)	(301,036)
Net	651,668	604,277	1,931,476	1,816,795
Premiums earned:
Direct	706,497	666,048	2,086,953	1,967,364
Assumed	6,484	7,623	19,220	19,465
Ceded	(98,704)	(101,616)	(295,232)	(285,890)
Net	614,277	572,055	1,810,941	1,700,939
Loss and loss expenses incurred:
Direct	477,427	455,728	1,289,357	1,187,400
Assumed	6,529	5,420	16,897	17,623
Ceded	(104,757)	(116,561)	(175,786)	(201,405)
Net	379,199	344,587	1,130,468	1,003,618
National Flood Insurance Program [Member]
Premiums written:
Ceded	(70,100)	(68,132)	(193,110)	(188,274)
Premiums earned:
Ceded	(61,448)	(59,847)	(180,582)	(174,779)
Loss and loss expenses incurred:
Ceded	$ (89,396)	$ (112,994)	$ (115,376)	$ (134,675)

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