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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) | 12 Months Ended |
--- | --- | ---
Dec. 31, 2017 USD ($) security | Dec. 31, 2016 USD ($) security
Fair Value |  |
Less than 12 Months | $ 1,080,300,000 | $ 1,063,400,000
12 Months or More | 705,300,000 | 15,800,000
Total | 1,785,600,000 | 1,079,200,000
Gross Unrealized Losses |  |
Less than 12 Months | (10,700,000) | (19,500,000)
12 Months or More | (13,800,000) | (500,000)
Total | (24,500,000) | (20,000,000)
Fair Value |  |
Less than 12 Months | 180,600,000 | 203,400,000
12 Months or More | 87,700,000 | 22,800,000
Total | 268,300,000 | 226,200,000
Gross Unrealized Losses |  |
Less than 12 Months | (9,700,000) | (8,800,000)
12 Months or More | (3,000,000) | (1,300,000)
Total | $ (12,700,000) | $ (10,100,000)
Available-for-Sale and Held-to-Maturity: |  |
Investment securities in an unrealized loss position (number of securities) | security | 581 | 396
Impairment losses | $ 0 |
Obligations of U.S. government agencies |  |
Fair Value |  |
Less than 12 Months | 284,900,000 | $ 316,500,000
12 Months or More | 266,100,000 | 0
Total | 551,000,000 | 316,500,000
Gross Unrealized Losses |  |
Less than 12 Months | (3,400,000) | (6,400,000)
12 Months or More | (4,600,000) | 0
Total | (8,000,000) | (6,400,000)
Fair Value |  |
Less than 12 Months | 9,700,000 | 19,600,000
12 Months or More | 9,900,000 | 0
Total | 19,600,000 | 19,600,000
Gross Unrealized Losses |  |
Less than 12 Months | 0 | (200,000)
12 Months or More | (200,000) | 0
Total | (200,000) | (200,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations |  |
Fair Value |  |
Less than 12 Months | 670,100,000 | 746,300,000
12 Months or More | 439,200,000 | 15,800,000
Total | 1,109,300,000 | 762,100,000
Gross Unrealized Losses |  |
Less than 12 Months | (6,200,000) | (13,100,000)
12 Months or More | (9,200,000) | (500,000)
Total | (15,400,000) | (13,600,000)
Fair Value |  |
Less than 12 Months | 76,400,000 | 108,800,000
12 Months or More | 60,500,000 | 20,000,000
Total | 136,900,000 | 128,800,000
Gross Unrealized Losses |  |
Less than 12 Months | (9,100,000) | (7,900,000)
12 Months or More | (2,500,000) | (1,300,000)
Total | (11,600,000) | (9,200,000)
Private mortgage-backed securities |  |
Fair Value |  |
Less than 12 Months | 74,000,000 |
12 Months or More | 0 |
Total | 74,000,000 |
Gross Unrealized Losses |  |
Less than 12 Months | (800,000) |
12 Months or More | 0 |
Total | (800,000) |
Corporate securities |  |
Fair Value |  |
Less than 12 Months | 51,300,000 |
12 Months or More | 0 |
Total | 51,300,000 |
Gross Unrealized Losses |  |
Less than 12 Months | (300,000) |
12 Months or More | 0 |
Total | (300,000) |
Fair Value |  |
Less than 12 Months | 41,200,000 | 32,500,000
12 Months or More | 5,000,000 | 0
Total | 46,200,000 | 32,500,000
Gross Unrealized Losses |  |
Less than 12 Months | (200,000) | (200,000)
12 Months or More | (100,000) | 0
Total | (300,000) | (200,000)
US Treasury notes |  |
Fair Value |  |
Less than 12 Months |  | 600,000
12 Months or More |  | 0
Total |  | 600,000
Gross Unrealized Losses |  |
Less than 12 Months |  | 0
12 Months or More |  | 0
Total |  | 0
State, county and municipal securities |  |
Fair Value |  |
Less than 12 Months | 53,300,000 | 42,500,000
12 Months or More | 12,300,000 | 2,800,000
Total | 65,600,000 | 45,300,000
Gross Unrealized Losses |  |
Less than 12 Months | (400,000) | (500,000)
12 Months or More | (200,000) | 0
Total | $ (600,000) | $ (500,000)

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