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Investment Securities - Gross Unrealized Losses and Fair Values of Investment Securities (Details) | 3 Months Ended | 6 Months Ended |
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Jun. 30, 2018 USD ($) security | Jun. 30, 2017 USD ($) | Jun. 30, 2018 USD ($) security | Jun. 30, 2017 USD ($) | Dec. 31, 2017 USD ($) security
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | $ 1,266,200,000 |  | $ 1,266,200,000 |  | $ 1,080,300,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (26,400,000) |  | (26,400,000) |  | (10,700,000)
Available-for-Sale, 12 Months or More, Fair Value | 712,200,000 |  | 712,200,000 |  | 705,300,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (23,500,000) |  | (23,500,000) |  | (13,800,000)
Available-for-Sale, Total Fair Value | 1,978,400,000 |  | 1,978,400,000 |  | 1,785,600,000
Available-for-Sale, Total Gross Unrealized Losses | (49,900,000) |  | (49,900,000) |  | (24,500,000)
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 194,600,000 |  | 194,600,000 |  | 180,600,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (13,400,000) |  | (13,400,000) |  | (9,700,000)
Held-to-Maturity, 12 Months or More, Fair Value | 76,800,000 |  | 76,800,000 |  | 87,700,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (4,100,000) |  | (4,100,000) |  | (3,000,000)
Held-to-Maturity, Total Fair Value | 271,400,000 |  | 271,400,000 |  | 268,300,000
Held-to-Maturity, Total Gross Unrealized Losses | $ (17,500,000) |  | $ (17,500,000) |  | $ (12,700,000)
Investment securities in an unrealized loss position (securities) | security | 701 |  | 701 |  | 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 | 266,100,000 |  | 266,100,000 |  | $ 284,900,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (6,500,000) |  | (6,500,000) |  | (3,400,000)
Available-for-Sale, 12 Months or More, Fair Value | 272,100,000 |  | 272,100,000 |  | 266,100,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (7,300,000) |  | (7,300,000) |  | (4,600,000)
Available-for-Sale, Total Fair Value | 538,200,000 |  | 538,200,000 |  | 551,000,000
Available-for-Sale, Total Gross Unrealized Losses | (13,800,000) |  | (13,800,000) |  | (8,000,000)
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 34,800,000 |  | 34,800,000 |  | 76,400,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (600,000) |  | (600,000) |  | (9,100,000)
Held-to-Maturity, 12 Months or More, Fair Value | 4,900,000 |  | 4,900,000 |  | 60,500,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (100,000) |  | (100,000) |  | (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 | (700,000) |  | (700,000) |  | (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 | 905,500,000 |  | 905,500,000 |  | 670,100,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (17,800,000) |  | (17,800,000) |  | (6,200,000)
Available-for-Sale, 12 Months or More, Fair Value | 415,700,000 |  | 415,700,000 |  | 439,200,000
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (15,600,000) |  | (15,600,000) |  | (9,200,000)
Available-for-Sale, Total Fair Value | 1,321,200,000 |  | 1,321,200,000 |  | 1,109,300,000
Available-for-Sale, Total Gross Unrealized Losses | (33,400,000) |  | (33,400,000) |  | (15,400,000)
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 86,100,000 |  | 86,100,000 |  | 41,200,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (11,800,000) |  | (11,800,000) |  | (200,000)
Held-to-Maturity, 12 Months or More, Fair Value | 44,800,000 |  | 44,800,000 |  | 5,000,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (3,400,000) |  | (3,400,000) |  | (100,000)
Held-to-Maturity, Total Fair Value | 130,900,000 |  | 130,900,000 |  | 46,200,000
Held-to-Maturity, Total Gross Unrealized Losses | (15,200,000) |  | (15,200,000) |  | (300,000)
Private mortgage-backed securities |  |  |  |  |
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 54,000,000 |  | 54,000,000 |  | 74,000,000
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (1,100,000) |  | (1,100,000) |  | (800,000)
Available-for-Sale, 12 Months or More, Fair Value | 24,400,000 |  | 24,400,000 |  | 0
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | (600,000) |  | (600,000) |  | 0
Available-for-Sale, Total Fair Value | 78,400,000 |  | 78,400,000 |  | 74,000,000
Available-for-Sale, Total Gross Unrealized Losses | (1,700,000) |  | (1,700,000) |  | (800,000)
State, county and municipal securities |  |  |  |  |
Held-to-Maturity: |  |  |  |  |
Held-to-Maturity, Less than 12 Months, Fair Value | 64,100,000 |  | 64,100,000 |  | 53,300,000
Held-to-Maturity, Less Than 12 Months, Gross Unrealized Losses | (900,000) |  | (900,000) |  | (400,000)
Held-to-Maturity, 12 Months or More, Fair Value | 17,300,000 |  | 17,300,000 |  | 12,300,000
Held-to-Maturity, 12 Months or More, Gross Unrealized Losses | (400,000) |  | (400,000) |  | (200,000)
Held-to-Maturity, Total Fair Value | 81,400,000 |  | 81,400,000 |  | 65,600,000
Held-to-Maturity, Total Gross Unrealized Losses | (1,300,000) |  | (1,300,000) |  | (600,000)
Corporate securities |  |  |  |  |
Available-for-Sale: |  |  |  |  |
Available-for-Sale, Less than 12 Months, Fair Value | 40,600,000 |  | 40,600,000 |  |
Available-for-sale, Less than 12 Months, Gross Unrealized Losses | (1,000,000) |  | (1,000,000) |  |
Available-for-Sale, 12 Months or More, Fair Value | 0 |  | 0 |  |
Available-for-Sale, 12 Months or More, Gross Unrealized Losses | 0 |  | 0 |  |
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 | (100,000) |  | (100,000) |  | 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 | (200,000) |  | (200,000) |  | (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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