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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) | 3 Months Ended | 9 Months Ended |
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Sep. 30, 2018 USD ($) security | Sep. 30, 2017 USD ($) | Sep. 30, 2018 USD ($) security | Sep. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) security
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | $ 1,140,300,000 |  | $ 1,140,300,000 |  | $ 1,080,300,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (26,100,000) |  | (26,100,000) |  | (10,700,000)
Available-for-Sale, 12 Months or More, Fair Value | 819,500,000 |  | 819,500,000 |  | 705,300,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (29,800,000) |  | (29,800,000) |  | (13,800,000)
Available-for-Sale, Total Fair Value | 1,959,800,000 |  | 1,959,800,000 |  | 1,785,600,000
Available-for-Sale, Total Gross Unrealized Losses | (55,900,000) |  | (55,900,000) |  | (24,500,000)
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 186,100,000 |  | 186,100,000 |  | 180,600,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (14,700,000) |  | (14,700,000) |  | (9,700,000)
Held-to-Maturity, 12 Months or More, Fair Value | 88,800,000 |  | 88,800,000 |  | 87,700,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (4,700,000) |  | (4,700,000) |  | (3,000,000)
Held-to-Maturity, Total Fair Value | 274,900,000 |  | 274,900,000 |  | 268,300,000
Held-to-Maturity, Total Gross Unrealized Losses | $ (19,400,000) |  | $ (19,400,000) |  | $ (12,700,000)
Investment securities in an unrealized loss position (securities) | security | 720 |  | 720 |  | 581
Impairment loss | $ 0 | $ 0 | $ 0 | $ 0 |
Obligations of U.S. government agencies |  |  |  |  |
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 199,900,000 |  | 199,900,000 |  | $ 284,900,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (4,800,000) |  | (4,800,000) |  | (3,400,000)
Available-for-Sale, 12 Months or More, Fair Value | 346,500,000 |  | 346,500,000 |  | 266,100,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (10,500,000) |  | (10,500,000) |  | (4,600,000)
Available-for-Sale, Total Fair Value | 546,400,000 |  | 546,400,000 |  | 551,000,000
Available-for-Sale, Total Gross Unrealized Losses | (15,300,000) |  | (15,300,000) |  | (8,000,000)
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 30,900,000 |  | 30,900,000 |  | 76,400,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (400,000) |  | (400,000) |  | (9,100,000)
Held-to-Maturity, 12 Months or More, Fair Value | 8,800,000 |  | 8,800,000 |  | 60,500,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (200,000) |  | (200,000) |  | (2,500,000)
Held-to-Maturity, Total Fair Value | 39,700,000 |  | 39,700,000 |  | 136,900,000
Held-to-Maturity, Total Gross Unrealized Losses | (600,000) |  | (600,000) |  | (11,600,000)
U.S. agency residential mortgage-backed securities & collateralized mortgage obligations |  |  |  |  |
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 851,800,000 |  | 851,800,000 |  | 670,100,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (19,200,000) |  | (19,200,000) |  | (6,200,000)
Available-for-Sale, 12 Months or More, Fair Value | 448,200,000 |  | 448,200,000 |  | 439,200,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (18,600,000) |  | (18,600,000) |  | (9,200,000)
Available-for-Sale, Total Fair Value | 1,300,000,000 |  | 1,300,000,000 |  | 1,109,300,000
Available-for-Sale, Total Gross Unrealized Losses | (37,800,000) |  | (37,800,000) |  | (15,400,000)
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 85,800,000 |  | 85,800,000 |  | 41,200,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (13,100,000) |  | (13,100,000) |  | (200,000)
Held-to-Maturity, 12 Months or More, Fair Value | 44,000,000 |  | 44,000,000 |  | 5,000,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (3,700,000) |  | (3,700,000) |  | (100,000)
Held-to-Maturity, Total Fair Value | 129,800,000 |  | 129,800,000 |  | 46,200,000
Held-to-Maturity, Total Gross Unrealized Losses | (16,800,000) |  | (16,800,000) |  | (300,000)
Private mortgage-backed securities |  |  |  |  |
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 49,900,000 |  | 49,900,000 |  | 74,000,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (1,200,000) |  | (1,200,000) |  | (800,000)
Available-for-Sale, 12 Months or More, Fair Value | 22,900,000 |  | 22,900,000 |  | 0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (600,000) |  | (600,000) |  | 0
Available-for-Sale, Total Fair Value | 72,800,000 |  | 72,800,000 |  | 74,000,000
Available-for-Sale, Total Gross Unrealized Losses | (1,800,000) |  | (1,800,000) |  | (800,000)
State, county and municipal securities |  |  |  |  |
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 59,800,000 |  | 59,800,000 |  | 53,300,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (1,000,000) |  | (1,000,000) |  | (400,000)
Held-to-Maturity, 12 Months or More, Fair Value | 26,200,000 |  | 26,200,000 |  | 12,300,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (700,000) |  | (700,000) |  | (200,000)
Held-to-Maturity, Total Fair Value | 86,000,000 |  | 86,000,000 |  | 65,600,000
Held-to-Maturity, Total Gross Unrealized Losses | (1,700,000) |  | (1,700,000) |  | (600,000)
Corporate securities |  |  |  |  |
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 38,700,000 |  | 38,700,000 |  |
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (900,000) |  | (900,000) |  |
Available-for-Sale, 12 Months or More, Fair Value | 1,900,000 |  | 1,900,000 |  |
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (100,000) |  | (100,000) |  |
Available-for-Sale, Total Fair Value | 40,600,000 |  | 40,600,000 |  |
Available-for-Sale, Total Gross Unrealized Losses | (1,000,000) |  | (1,000,000) |  |
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 9,600,000 |  | 9,600,000 |  | 9,700,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (200,000) |  | (200,000) |  | 0
Held-to-Maturity, 12 Months or More, Fair Value | 9,800,000 |  | 9,800,000 |  | 9,900,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (100,000) |  | (100,000) |  | (200,000)
Held-to-Maturity, Total Fair Value | 19,400,000 |  | 19,400,000 |  | 19,600,000
Held-to-Maturity, Total Gross Unrealized Losses | $ (300,000) |  | $ (300,000) |  | (200,000)
Other Investments |  |  |  |  |
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value |  |  |  |  | 51,300,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses |  |  |  |  | (300,000)
Available-for-Sale, 12 Months or More, Fair Value |  |  |  |  | 0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses |  |  |  |  | 0
Available-for-Sale, Total Fair Value |  |  |  |  | 51,300,000
Available-for-Sale, Total Gross Unrealized Losses |  |  |  |  | $ (300,000)

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