Supplemental Reinsurance Premiums (Schedule of Property and Liability Insurance Earned Premiums) (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Supplementary Insurance Information [Abstract]
Direct amounts									$ 2,970,424	$ 2,806,889	$ 2,704,401
Ceded to other companies									(12,964)	(11,185)	(7,059)
Assumed									437	491	845
Net amounts	$ 760,094	$ 745,520	$ 731,546	$ 720,737	$ 709,368	$ 705,237	$ 697,889	$ 683,701	$ 2,957,897	$ 2,796,195	$ 2,698,187

Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums earned.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-04.1)
-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 605
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.13(c))
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 235
-Section S99
-Paragraph 2
-Subparagraph (SX 210.12-17)
-URI http://asc.fasb.org/extlink&oid=34713774&loc=d3e574992-122915

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