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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, 2019 USD ($) Investments | Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($) Investments
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity | $ 92,300,000 | $ 281,100,000 | $ 406,800,000
Fair Value |  |  |
Less than 12 Months |  |  | 90,400,000
12 Months or More |  |  | 216,900,000
Gross Unrealized Losses |  |  |
Less than 12 Months |  |  | (2,800,000)
12 Months or More |  |  | (5,500,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  |  | 307,300,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  |  | $ (8,300,000)
Available-for-Sale and Held-to-Maturity: |  |  |
Investment securities in an unrealized loss position (number of securities) | Investments | 338 |  | 760
Impairment losses | $ 0 |  |
Gross Unrealized Losses |  |  |
Less than 12 Months | (5,400,000) |  | $ (22,600,000)
12 Months or More | 202,500,000 |  | 779,000,000.0
12 Months or More | (1,800,000) |  | (14,500,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 1,127,900,000 |  | 1,902,200,000
Total | (7,200,000) |  | (37,100,000)
Less than 12 Months | 925,400,000 |  | 1,123,200,000
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member] |  |  |
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity | 1,200,000 |  | 189,700,000
Fair Value |  |  |
Less than 12 Months |  |  | 45,000,000.0
12 Months or More |  |  | 120,200,000
Gross Unrealized Losses |  |  |
Less than 12 Months |  |  | (2,200,000)
12 Months or More |  |  | (4,300,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  |  | 165,200,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  |  | (6,500,000)
Gross Unrealized Losses |  |  |
Less than 12 Months | (4,600,000) |  | (14,500,000)
12 Months or More | 155,900,000 |  | 503,700,000
12 Months or More | (1,400,000) |  | (9,600,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 896,000,000.0 |  | 1,238,900,000
Total | (6,000,000.0) |  | (24,100,000)
Less than 12 Months | 740,100,000 |  | 735,200,000
US Government Agencies Debt Securities [Member] |  |  |
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity | 19,800,000 |  | 19,800,000
Fair Value |  |  |
Less than 12 Months |  |  | 19,500,000
12 Months or More |  |  | 0
Gross Unrealized Losses |  |  |
Less than 12 Months |  |  | (300,000)
12 Months or More |  |  | 0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  |  | 19,500,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  |  | (300,000)
Gross Unrealized Losses |  |  |
Less than 12 Months | (800,000) |  | (7,900,000)
12 Months or More | 0 |  | 154,500,000
12 Months or More | 0 |  | (2,300,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 185,300,000 |  | 517,600,000
Total | (800,000) |  | (10,200,000)
Less than 12 Months | 185,300,000 |  | 363,100,000
Corporate Debt Securities [Member] |  |  |
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity | 13,900,000 |  | 46,300,000
Fair Value |  |  |
Less than 12 Months |  |  | 0
12 Months or More |  |  | 39,600,000
Gross Unrealized Losses |  |  |
Less than 12 Months |  |  | 0
12 Months or More |  |  | (600,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  |  | 39,600,000
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  |  | (600,000)
Gross Unrealized Losses |  |  |
Less than 12 Months |  |  | (200,000)
12 Months or More |  |  | 51,400,000
12 Months or More |  |  | (800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value |  |  | 76,300,000
Total |  |  | (1,000,000.0)
Less than 12 Months |  |  | 24,900,000
Mortgage-backed Securities, Issued by Private Enterprises [Member] |  |  |
Gross Unrealized Losses |  |  |
Less than 12 Months | 0 |  | 0
12 Months or More | 46,600,000 |  | 69,400,000
12 Months or More | (400,000) |  | (1,800,000)
Available-for-sale Securities, Continuous Unrealized Loss Position, Fair Value | 46,600,000 |  | 69,400,000
Total | (400,000) |  | (1,800,000)
Less than 12 Months | 0 |  | 0
State, county and municipal securities |  |  |
Investments, Debt and Equity Securities [Abstract] |  |  |
Debt Securities, Held-to-maturity | $ 57,300,000 |  | 150,900,000
Fair Value |  |  |
Less than 12 Months |  |  | 25,900,000
12 Months or More |  |  | 57,100,000
Gross Unrealized Losses |  |  |
Less than 12 Months |  |  | (300,000)
12 Months or More |  |  | (600,000)
Debt Securities, Held-to-maturity, Unrealized Loss Position, Fair Value |  |  | 83,000,000.0
Debt Securities, Held-to-maturity, Unrealized Loss Position, Accumulated Loss |  |  | $ (900,000)

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