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Investments (Fair Value and Gross Pre-Tax Net Unrealized Unrecognized Loss of Securities By Length of Time) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Percent Of Securities of Amortized Cost In An Unrealized Unrecognized Loss Position |  | 1.00% | 1.00%
AFS Fair Value, Less than 12 months |  | $ 3,070,757 | $ 878,126
AFS Unrealized Losses, Less than 12 months | [1] | (47,981) | (4,961)
AFS Fair Value, 12 months or longer |  | 17,853 | 35,686
AFS Unrealized Losses, 12 months or longer | [1] | (652) | (397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 3,088,610 | 913,812
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | (48,633) | (5,358)
Held-to-maturity Securities [Member] |  |  |
Unrealized Unrecognized Loss |  | 300 | 100
AFS Fixed Income Securities [Member] |  |  |
AFS Fair Value, Less than 12 months |  |  | 869,566
AFS Unrealized Losses, Less than 12 months | [1] |  | (4,664)
AFS Fair Value, 12 months or longer |  |  | 35,686
AFS Unrealized Losses, 12 months or longer | [1] |  | (397)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  |  | 905,252
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] |  | (5,061)
AFS Fixed Income Securities [Member] | U.S. Government and Government Agencies [Member] |  |  |
AFS Fair Value, Less than 12 months |  | 28,005 | 23,516
AFS Unrealized Losses, Less than 12 months | [1] | (763) | (233)
AFS Fair Value, 12 months or longer |  | 0 | 250
AFS Unrealized Losses, 12 months or longer | [1] | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 28,005 | 23,766
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | (763) | (233)
AFS Fixed Income Securities [Member] | Foreign Government [Member] |  |  |
AFS Fair Value, Less than 12 months |  | 7,740 | 1,481
AFS Unrealized Losses, Less than 12 months | [1] | (129) | (11)
AFS Fair Value, 12 months or longer |  | 0 | 0
AFS Unrealized Losses, 12 months or longer | [1] | 0 | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 7,740 | 1,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | (129) | (11)
AFS Fixed Income Securities [Member] | Obligations of States and Political Subdivisions [Member] |  |  |
AFS Fair Value, Less than 12 months |  | 342,653 | 107,514
AFS Unrealized Losses, Less than 12 months | [1] | (4,316) | (422)
AFS Fair Value, 12 months or longer |  | 3,422 | 14,139
AFS Unrealized Losses, 12 months or longer | [1] | (126) | (160)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 346,075 | 121,653
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | (4,442) | (582)
AFS Fixed Income Securities [Member] | Corporate Securities [Member] |  |  |
AFS Fair Value, Less than 12 months |  | 1,147,312 | 238,326
AFS Unrealized Losses, Less than 12 months | [1] | (22,989) | (1,744)
AFS Fair Value, 12 months or longer |  | 2,762 | 3,228
AFS Unrealized Losses, 12 months or longer | [1] | (142) | (18)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 1,150,074 | 241,554
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | (23,131) | (1,762)
AFS Fixed Income Securities [Member] | Collateralized Loan Obligations and Other Asset Backed Securities [Member] |  |  |
AFS Fair Value, Less than 12 months |  | 427,307 | 74,977
AFS Unrealized Losses, Less than 12 months | [1] | (1,813) | (196)
AFS Fair Value, 12 months or longer |  | 774 | 1,655
AFS Unrealized Losses, 12 months or longer | [1] | (4) | (6)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 428,081 | 76,632
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | (1,817) | (202)
AFS Fixed Income Securities [Member] | Commercial Mortgage Backed Securities [Member] |  |  |
AFS Fair Value, Less than 12 months |  | 378,523 | 154,267
AFS Unrealized Losses, Less than 12 months | [1] | (6,009) | (773)
AFS Fair Value, 12 months or longer |  | 0 | 5,214
AFS Unrealized Losses, 12 months or longer | [1] | 0 | (68)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 378,523 | 159,481
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | (6,009) | (841)
AFS Fixed Income Securities [Member] | Residential Mortgage Backed Securities [Member] |  |  |
AFS Fair Value, Less than 12 months |  | 739,217 | 269,485
AFS Unrealized Losses, Less than 12 months | [1] | (11,962) | (1,285)
AFS Fair Value, 12 months or longer |  | 10,895 | 11,200
AFS Unrealized Losses, 12 months or longer | [1] | (380) | (145)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  | 750,112 | 280,685
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] | $ (12,342) | (1,430)
Equity Securities [Member] |  |  |
AFS Fair Value, Less than 12 months |  |  | 8,560
AFS Unrealized Losses, Less than 12 months | [1] |  | (297)
AFS Fair Value, 12 months or longer |  |  | 0
AFS Unrealized Losses, 12 months or longer | [1] |  | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  |  | 8,560
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] |  | (297)
Equity Securities [Member] | Common Stock [Member] |  |  |
AFS Fair Value, Less than 12 months |  |  | 4,727
AFS Unrealized Losses, Less than 12 months | [1] |  | (226)
AFS Fair Value, 12 months or longer |  |  | 0
AFS Unrealized Losses, 12 months or longer | [1] |  | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  |  | 4,727
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] |  | (226)
Equity Securities [Member] | Preferred Stock [Member] |  |  |
AFS Fair Value, Less than 12 months |  |  | 3,833
AFS Unrealized Losses, Less than 12 months | [1] |  | (71)
AFS Fair Value, 12 months or longer |  |  | 0
AFS Unrealized Losses, 12 months or longer | [1] |  | 0
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  |  | 3,833
Available-for-sale Securities, Continuous Unrealized Loss Position, Accumulated Loss | [1] |  | $ (71)
[1] | Gross unrealized losses include non-OTTI unrealized amounts and OTTI losses recognized in AOCI.

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