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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) | 12 Months Ended |
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Dec. 31, 2018 USD ($) Investments | Dec. 31, 2017 USD ($) Investments
Fair Value |  |
Less than 12 Months | $ 1,123,200,000 | $ 1,080,300,000
12 Months or More | 779,000,000 | 705,300,000
Total | 1,902,200,000 | 1,785,600,000
Gross Unrealized Losses |  |
Less than 12 Months | (22,600,000) | (10,700,000)
12 Months or More | (14,500,000) | (13,800,000)
Total | (37,100,000) | (24,500,000)
Fair Value |  |
Less than 12 Months | 90,400,000 | 180,600,000
12 Months or More | 216,900,000 | 87,700,000
Gross Unrealized Losses |  |
Less than 12 Months | (2,800,000) | (9,700,000)
12 Months or More | (5,500,000) | (3,000,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 307,300,000 | 268,300,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | $ (8,300,000) | $ (12,700,000)
Available-for-Sale and Held-to-Maturity: |  |
Investment securities in an unrealized loss position (number of securities) | Investments | 760 | 581
Impairment losses | $ 0 |
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations |  |
Fair Value |  |
Less than 12 Months | 735,200,000 | $ 670,100,000
12 Months or More | 503,700,000 | 439,200,000
Total | 1,238,900,000 | 1,109,300,000
Gross Unrealized Losses |  |
Less than 12 Months | (14,500,000) | (6,200,000)
12 Months or More | (9,600,000) | (9,200,000)
Total | (24,100,000) | (15,400,000)
Fair Value |  |
Less than 12 Months | 0 | 76,400,000
12 Months or More | 39,600,000 | 60,500,000
Gross Unrealized Losses |  |
Less than 12 Months | 0 | (9,100,000)
12 Months or More | (600,000) | (2,500,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 39,600,000 | 136,900,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (600,000) | (11,600,000)
Obligations of U.S. government agencies |  |
Fair Value |  |
Less than 12 Months | 363,100,000 | 284,900,000
12 Months or More | 154,500,000 | 266,100,000
Total | 517,600,000 | 551,000,000
Gross Unrealized Losses |  |
Less than 12 Months | (7,900,000) | (3,400,000)
12 Months or More | (2,300,000) | (4,600,000)
Total | (10,200,000) | (8,000,000)
Fair Value |  |
Less than 12 Months | 19,500,000 | 9,700,000
12 Months or More | 0 | 9,900,000
Gross Unrealized Losses |  |
Less than 12 Months | (300,000) | 0
12 Months or More | 0 | (200,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 19,500,000 | 19,600,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (300,000) | (200,000)
Corporate Debt Securities [Member] |  |
Fair Value |  |
Less than 12 Months | 24,900,000 | 51,300,000
12 Months or More | 51,400,000 | 0
Total | 76,300,000 | 51,300,000
Gross Unrealized Losses |  |
Less than 12 Months | (200,000) | (300,000)
12 Months or More | (800,000) | 0
Total | (1,000,000) | (300,000)
Fair Value |  |
Less than 12 Months | 45,000,000 | 41,200,000
12 Months or More | 120,200,000 | 5,000,000
Gross Unrealized Losses |  |
Less than 12 Months | (2,200,000) | (200,000)
12 Months or More | (4,300,000) | (100,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 165,200,000 | 46,200,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | (6,500,000) | (300,000)
Private mortgage-backed securities |  |
Fair Value |  |
Less than 12 Months | 0 | 74,000,000
12 Months or More | 69,400,000 | 0
Total | 69,400,000 | 74,000,000
Gross Unrealized Losses |  |
Less than 12 Months | 0 | (800,000)
12 Months or More | (1,800,000) | 0
Total | (1,800,000) | (800,000)
State, county and municipal securities |  |
Fair Value |  |
Less than 12 Months | 25,900,000 | 53,300,000
12 Months or More | 57,100,000 | 12,300,000
Gross Unrealized Losses |  |
Less than 12 Months | (300,000) | (400,000)
12 Months or More | (600,000) | (200,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value | 83,000,000 | 65,600,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss | $ (900,000) | $ (600,000)

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