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Securities (Continuous Unrealized Losses) (Details) | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) security | Mar. 31, 2017 USD ($) | Dec. 31, 2017 USD ($) security
Fair Value |  |  |
Less Than 12 Months | $ 1,045,676,000 |  | $ 942,845,000
12 Months or More | 1,444,107,000 |  | 1,483,868,000
Total | 2,489,783,000 |  | 2,426,713,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (22,728,000) |  | (9,919,000)
12 Months or More | (59,460,000) |  | (48,409,000)
Total | (82,188,000) |  | $ (58,328,000)
Impairment loss | $ 0 | $ 0 |
Number of securities in an unrealized loss position | security | 192 |  | 165
U.S. Treasury securities |  |  |
Fair Value |  |  |
Less Than 12 Months | $ 170,103,000 |  | $ 168,061,000
12 Months or More | 457,952,000 |  | 472,219,000
Total | 628,055,000 |  | 640,280,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (3,130,000) |  | (1,005,000)
12 Months or More | (13,803,000) |  | (10,110,000)
Total | $ (16,933,000) |  | $ (11,115,000)
Number of securities in an unrealized loss position | security | 23 |  | 25
U.S. government agency and U.S. government sponsored enterprise debt securities |  |  |
Fair Value |  |  |
Less Than 12 Months | $ 133,861,000 |  | $ 99,935,000
12 Months or More | 86,549,000 |  | 85,281,000
Total | 220,410,000 |  | 185,216,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (2,157,000) |  | (623,000)
12 Months or More | (778,000) |  | (2,862,000)
Total | $ (2,935,000) |  | $ (3,485,000)
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 | 111 |  | 98
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Commercial mortgage-backed securities |  |  |
Fair Value |  |  |
Less Than 12 Months | $ 117,057,000 |  | $ 113,775,000
12 Months or More | 192,874,000 |  | 191,827,000
Total | 309,931,000 |  | 305,602,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (3,255,000) |  | (2,071,000)
12 Months or More | (10,061,000) |  | (7,461,000)
Total | (13,316,000) |  | (9,532,000)
U.S. government agency and U.S. government sponsored enterprise mortgage-backed securities - Residential mortgage-backed securities |  |  |
Fair Value |  |  |
Less Than 12 Months | 461,197,000 |  | 413,621,000
12 Months or More | 339,157,000 |  | 361,809,000
Total | 800,354,000 |  | 775,430,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (8,362,000) |  | (4,205,000)
12 Months or More | (12,264,000) |  | (9,357,000)
Total | (20,626,000) |  | (13,562,000)
Municipal securities |  |  |
Fair Value |  |  |
Less Than 12 Months | 24,618,000 |  | 8,490,000
12 Months or More | 8,323,000 |  | 8,588,000
Total | 32,941,000 |  | 17,078,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (671,000) |  | (123,000)
12 Months or More | (446,000) |  | (186,000)
Total | (1,117,000) |  | (309,000)
Non-agency residential mortgage-backed securities | Investment grade |  |  |
Fair Value |  |  |
Less Than 12 Months | 8,404,000 |  | 4,599,000
12 Months or More | 0 |  | 0
Total | 8,404,000 |  | 4,599,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (143,000) |  | (22,000)
12 Months or More | 0 |  | 0
Total | (143,000) |  | (22,000)
Corporate debt securities | Investment grade |  |  |
Fair Value |  |  |
Less Than 12 Months | 0 |  | 0
12 Months or More | 10,742,000 |  | 11,905,000
Total | 10,742,000 |  | 11,905,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | 0 |  | 0
12 Months or More | (699,000) |  | (746,000)
Total | (699,000) |  | (746,000)
Foreign bonds | Investment grade |  |  |
Fair Value |  |  |
Less Than 12 Months | 130,436,000 |  | 103,149,000
12 Months or More | 348,510,000 |  | 352,239,000
Total | 478,946,000 |  | 455,388,000
Gross Unrealized Losses |  |  |
Less Than 12 Months | (5,010,000) |  | (1,325,000)
12 Months or More | (21,409,000) |  | (17,687,000)
Total | $ (26,419,000) |  | $ (19,012,000)
Number of securities in an unrealized loss position | security | 17 |  | 16
Other securities |  |  |
Fair Value |  |  |
Less Than 12 Months |  |  | $ 31,215,000
12 Months or More |  |  | 0
Total |  |  | 31,215,000
Gross Unrealized Losses |  |  |
Less Than 12 Months |  |  | (545,000)
12 Months or More |  |  | 0
Total |  |  | $ (545,000)

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