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Investments, Investments Temporarily Impaired (Details)
$ in Thousands
Dec. 31, 2017
USD ($)
Investment
Temporarily Impaired Investments [Abstract]  
Number of investments | Investment 588
Estimated fair value, less than 12 months of temporary impairment $ 752,571
Unrealized losses, less than 12 months of temporary impairment (2,062)
Estimated fair value, more than 12 months of temporary impairment 127,097
Unrealized losses, more than 12 months of temporary impairment (1,010)
Estimated fair value, total temporary impairment 879,668
Unrealized losses, total temporary impairment $ (3,072)
Corporate Debt Securities [Member]  
Temporarily Impaired Investments [Abstract]  
Number of investments | Investment 476
Estimated fair value, less than 12 months of temporary impairment $ 551,446
Unrealized losses, less than 12 months of temporary impairment (1,236)
Estimated fair value, more than 12 months of temporary impairment 74,987
Unrealized losses, more than 12 months of temporary impairment (575)
Estimated fair value, total temporary impairment 626,433
Unrealized losses, total temporary impairment $ (1,811)
Debt Securities issued by U.S. Government Agencies [Member]  
Temporarily Impaired Investments [Abstract]  
Number of investments | Investment 45
Estimated fair value, less than 12 months of temporary impairment $ 107,788
Unrealized losses, less than 12 months of temporary impairment (262)
Estimated fair value, more than 12 months of temporary impairment 27,538
Unrealized losses, more than 12 months of temporary impairment (118)
Estimated fair value, total temporary impairment 135,326
Unrealized losses, total temporary impairment $ (380)
Debt Securities issued by the U.S. Treasury [Member]  
Temporarily Impaired Investments [Abstract]  
Number of investments | Investment 7
Estimated fair value, less than 12 months of temporary impairment $ 30,818
Unrealized losses, less than 12 months of temporary impairment (19)
Estimated fair value, more than 12 months of temporary impairment 0
Unrealized losses, more than 12 months of temporary impairment 0
Estimated fair value, total temporary impairment 30,818
Unrealized losses, total temporary impairment $ (19)
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member]  
Temporarily Impaired Investments [Abstract]  
Number of investments | Investment 60
Estimated fair value, less than 12 months of temporary impairment $ 62,519
Unrealized losses, less than 12 months of temporary impairment (545)
Estimated fair value, more than 12 months of temporary impairment 24,572
Unrealized losses, more than 12 months of temporary impairment (317)
Estimated fair value, total temporary impairment 87,091
Unrealized losses, total temporary impairment $ (862)