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Securities (Continuous Unrealized Losses) (Details) | 12 Months Ended
--- | ---
Dec. 31, 2017 USD ($) security | Dec. 31, 2016 USD ($) security | Dec. 31, 2015 USD ($)
Fair Value |  |  |
Less Than 12 Months | $ 942,845,000 | $ 2,162,896,000 |
12 Months or More | 1,483,868,000 | 253,038,000 |
Total | 2,426,713,000 | 2,415,934,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (9,919,000) | (51,760,000) |
12 Months or More | (48,409,000) | (4,581,000) |
Total | (58,328,000) | (56,341,000) |
Impairment loss on available-for-sale securities | $ 0 | $ 0 | $ 0
Number of securities in an unrealized loss position | security | 165 | 170 |
Fair Value |  |  |
Less Than 12 Months | $ 942,845,000 | $ 2,162,896,000 |
12 Months or More | 1,483,868,000 | 253,038,000 |
Total | 2,426,713,000 | 2,415,934,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (9,919,000) | (51,760,000) |
12 Months or More | (48,409,000) | (4,581,000) |
Total | (58,328,000) | (56,341,000) |
U.S. Treasury securities |  |  |
Fair Value |  |  |
Less Than 12 Months | 168,061,000 | 670,268,000 |
12 Months or More | 472,219,000 | 0 |
Total | 640,280,000 | 670,268,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (1,005,000) | (9,829,000) |
12 Months or More | (10,110,000) | 0 |
Total | $ (11,115,000) | $ (9,829,000) |
Number of securities in an unrealized loss position | security | 25 | 26 |
U.S. government agency and U.S. government sponsored enterprise debt securities |  |  |
Fair Value |  |  |
Less Than 12 Months | $ 99,935,000 | $ 203,901,000 |
12 Months or More | 85,281,000 | 0 |
Total | 185,216,000 | 203,901,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (623,000) | (3,249,000) |
12 Months or More | (2,862,000) | 0 |
Total | $ (3,485,000) | $ (3,249,000) |
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities |  |  |
Gross Unrealized Losses |  |  |
Number of securities in an unrealized loss position | security | 98 | 82 |
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities |  |  |
Fair Value |  |  |
Less Than 12 Months | $ 113,775,000 | $ 202,106,000 |
12 Months or More | 191,827,000 | 29,201,000 |
Total | 305,602,000 | 231,307,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (2,071,000) | (5,452,000) |
12 Months or More | (7,461,000) | (766,000) |
Total | (9,532,000) | (6,218,000) |
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities |  |  |
Fair Value |  |  |
Less Than 12 Months | 413,621,000 | 629,324,000 |
12 Months or More | 361,809,000 | 119,603,000 |
Total | 775,430,000 | 748,927,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (4,205,000) | (9,594,000) |
12 Months or More | (9,357,000) | (1,955,000) |
Total | (13,562,000) | (11,549,000) |
Municipal securities |  |  |
Fair Value |  |  |
Less Than 12 Months | 8,490,000 | 57,655,000 |
12 Months or More | 8,588,000 | 2,692,000 |
Total | 17,078,000 | 60,347,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (123,000) | (1,699,000) |
12 Months or More | (186,000) | (201,000) |
Total | (309,000) | (1,900,000) |
Non-agency residential mortgage-backed securities | Investment grade |  |  |
Fair Value |  |  |
Less Than 12 Months | 4,599,000 | 5,033,000 |
12 Months or More | 0 | 6,444,000 |
Total | 4,599,000 | 11,477,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (22,000) | (101,000) |
12 Months or More | 0 | (14,000) |
Total | (22,000) | (115,000) |
Corporate debt securities | Investment grade |  |  |
Fair Value |  |  |
Less Than 12 Months | 0 | 0 |
12 Months or More | 11,905,000 | 71,667,000 |
Total | 11,905,000 | 71,667,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | 0 | 0 |
12 Months or More | (746,000) | (375,000) |
Total | (746,000) | (375,000) |
Corporate debt securities | Non-investment grade |  |  |
Fair Value |  |  |
Less Than 12 Months |  | 0 |
12 Months or More |  | 9,173,000 |
Total |  | 9,173,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months |  | 0 |
12 Months or More |  | (1,018,000) |
Total |  | (1,018,000) |
Foreign bonds | Investment grade |  |  |
Fair Value |  |  |
Less Than 12 Months | 103,149,000 | 363,618,000 |
12 Months or More | 352,239,000 | 14,258,000 |
Total | 455,388,000 | 377,876,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (1,325,000) | (21,327,000) |
12 Months or More | (17,687,000) | (252,000) |
Total | $ (19,012,000) | $ (21,579,000) |
Number of securities in an unrealized loss position | security | 16 | 13 |
Other securities |  |  |
Fair Value |  |  |
Less Than 12 Months | $ 31,215,000 | $ 30,991,000 |
12 Months or More | 0 | 0 |
Total | 31,215,000 | 30,991,000 |
Gross Unrealized Losses |  |  |
Less Than 12 Months | (545,000) | (509,000) |
12 Months or More | 0 | 0 |
Total | (545,000) | (509,000) |
Non-agency commercial mortgage-backed security |  |  |
Fair Value |  |  |
Less Than 12 Months | 0 | 0 |
12 months or More | 0 | 0 |
Total | 0 | 0 |
Gross Unrealized Losses |  |  |
Less than 12 months | 0 | 0 |
12 Months or More | 0 | 0 |
Total | $ 0 | $ 0 |

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