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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) | 3 Months Ended |  |
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Mar. 31, 2019 USD ($) Investments | Mar. 31, 2018 USD ($) | Dec. 31, 2018 USD ($) Investments | Dec. 31, 2017 USD ($)
Available-for-Sale: |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | $ 872,700,000 |  |  | $ 1,123,200,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (9,700,000) |  |  | (22,600,000)
Available-for-Sale, 12 Months or More, Fair Value | 697,600,000 |  |  | 779,000,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (8,000,000) |  |  | (14,500,000)
Available-for-Sale, Total Fair Value | 1,570,300,000 |  |  | 1,902,200,000
Available-for-Sale, Total Gross Unrealized Losses | $ (17,700,000) |  |  | (37,100,000)
Held-to-Maturity: |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value |  |  | $ 28,600,000 | 90,400,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses |  |  | (300,000) | (2,800,000)
Held-to-Maturity, 12 Months or More, Fair Value |  |  | 44,600,000 | 216,900,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses |  |  | (200,000) | (5,500,000)
Held-to-Maturity, Total Fair Value |  |  | 73,200,000 | 307,300,000
Held-to-Maturity, Total Gross Unrealized Losses |  |  | $ (500,000) | (8,300,000)
Investment securities in an unrealized loss position (securities) | Investments | 551 |  | 760 |
Impairment loss | $ 0 | $ 0 |  |
Available-for-sale Securities, Gross Realized Gain (Loss) | 0 | $ 0 |  |
Obligations of U.S. government agencies |  |  |  |
Available-for-Sale: |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 336,800,000 |  |  | 363,100,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (2,800,000) |  |  | (7,900,000)
Available-for-Sale, 12 Months or More, Fair Value | 129,700,000 |  |  | 154,500,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (1,200,000) |  |  | (2,300,000)
Available-for-Sale, Total Fair Value | 466,500,000 |  |  | 517,600,000
Available-for-Sale, Total Gross Unrealized Losses | (4,000,000) |  |  | (10,200,000)
Held-to-Maturity: |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value |  |  | $ 0 | 19,500,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses |  |  | 0 | (300,000)
Held-to-Maturity, 12 Months or More, Fair Value |  |  | 28,000,000 | 0
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses |  |  | (100,000) | 0
Held-to-Maturity, Total Fair Value |  |  | 28,000,000 | 19,500,000
Held-to-Maturity, Total Gross Unrealized Losses |  |  | (100,000) | (300,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations |  |  |  |
Available-for-Sale: |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 535,900,000 |  |  | 735,200,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (6,900,000) |  |  | (14,500,000)
Available-for-Sale, 12 Months or More, Fair Value | 450,300,000 |  |  | 503,700,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (5,500,000) |  |  | (9,600,000)
Available-for-Sale, Total Fair Value | 986,200,000 |  |  | 1,238,900,000
Available-for-Sale, Total Gross Unrealized Losses | (12,400,000) |  |  | (24,100,000)
Held-to-Maturity: |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value |  |  |  | 45,000,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses |  |  |  | (2,200,000)
Held-to-Maturity, 12 Months or More, Fair Value |  |  |  | 120,200,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses |  |  |  | (4,300,000)
Held-to-Maturity, Total Fair Value |  |  |  | 165,200,000
Held-to-Maturity, Total Gross Unrealized Losses |  |  |  | (6,500,000)
Private mortgage-backed securities |  |  |  |
Available-for-Sale: |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 0 |  |  | 0
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | 0 |  |  | 0
Available-for-Sale, 12 Months or More, Fair Value | 64,500,000 |  |  | 69,400,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (900,000) |  |  | (1,800,000)
Available-for-Sale, Total Fair Value | 64,500,000 |  |  | 69,400,000
Available-for-Sale, Total Gross Unrealized Losses | (900,000) |  |  | (1,800,000)
State, county and municipal securities |  |  |  |
Available-for-Sale: |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 0 |  |  |
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | 0 |  |  |
Available-for-Sale, 12 Months or More, Fair Value | 16,200,000 |  |  |
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (100,000) |  |  |
Available-for-Sale, Total Fair Value | 16,200,000 |  |  |
Available-for-Sale, Total Gross Unrealized Losses | (100,000) |  |  |
Held-to-Maturity: |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value |  |  | 9,000,000 | 25,900,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses |  |  | (100,000) | (300,000)
Held-to-Maturity, 12 Months or More, Fair Value |  |  | 16,600,000 | 57,100,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses |  |  | (100,000) | (600,000)
Held-to-Maturity, Total Fair Value |  |  | 25,600,000 | 83,000,000
Held-to-Maturity, Total Gross Unrealized Losses |  |  | (200,000) | (900,000)
Corporate securities |  |  |  |
Available-for-Sale: |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 0 |  |  | 24,900,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | 0 |  |  | (200,000)
Available-for-Sale, 12 Months or More, Fair Value | 36,900,000 |  |  | 51,400,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (300,000) |  |  | (800,000)
Available-for-Sale, Total Fair Value | 36,900,000 |  |  | 76,300,000
Available-for-Sale, Total Gross Unrealized Losses | $ (300,000) |  |  | (1,000,000)
Held-to-Maturity: |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value |  |  | 19,600,000 | 0
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses |  |  | (200,000) | 0
Held-to-Maturity, 12 Months or More, Fair Value |  |  | 0 | 39,600,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses |  |  | 0 | (600,000)
Held-to-Maturity, Total Fair Value |  |  | 19,600,000 | 39,600,000
Held-to-Maturity, Total Gross Unrealized Losses |  |  | $ (200,000) | $ (600,000)

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