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Investment Securities and Equity Securities with Readily Determinable Fair Values (Fair Value and Gross Unrealized Loss) (Details) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
Available for sale: |  |
Fair value, less than 12 months | $ 1,016,021 |
Marketable securities, less than 12 months |  | $ 1,078,610
Unrealized losses, less than 12 Months | (23,849) |
Marketable securities, less than 12 Months |  | (13,417)
Fair value, 12 months or more | 2,047,918 |
Marketable securities, 12 months or more |  | 2,037,996
Unrealized losses, 12 Months or More | 91,783 |
Marketable securities |  | 53,050
Fair value, Total | 3,063,939 |
Marketable securities, Total |  | 3,116,606
Unrealized losses, Total | 115,632 |
Marketable securities, Total |  | 66,467
Residential mortgage-backed securities |  |
Available for sale: |  |
Fair value, less than 12 months | 1,000,142 | 1,061,577
Unrealized losses, less than 12 Months | (23,712) | (13,157)
Fair value, 12 months or more | 2,047,402 | 2,029,455
Unrealized losses, 12 Months or More | 91,775 | 52,812
Fair value, Total | 3,047,544 | 3,091,032
Unrealized losses, Total | 115,487 | 65,969
Obligations of states and political subdivisions |  |
Available for sale: |  |
Fair value, less than 12 months | 15,879 | 5,534
Unrealized losses, less than 12 Months | (137) | (9)
Fair value, 12 months or more | 516 | 522
Unrealized losses, 12 Months or More | 8 | 7
Fair value, Total | 16,395 | 6,056
Unrealized losses, Total | $ 145 | 16
Equity securities |  |
Available for sale: |  |
Equity Securities, less than 12 months |  | 11,499
Equity Securities, less than 12 Months |  | (251)
Equity Securities, 12 months or more |  | 8,019
Equity Securities |  | 231
Equity Securities, Total |  | 19,518
Equity securities |  | $ 482

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