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Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | $ 1,474,517 | $ 1,220,992
Less than 12 Months Unrealized Losses | (32,847) | (9,442)
12 Months or More Fair Value | 909,144 | 948,366
12 Months or More Unrealized Losses | (40,615) | (24,608)
Total Fair Value | 2,383,661 | 2,169,358
Total Unrealized Losses | (73,462) | (34,050)
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | 906,188 | 816,678
Less than 12 Months Unrealized Losses | (22,365) | (6,710)
12 Months or More Fair Value | 722,422 | 717,211
12 Months or More Unrealized Losses | (33,989) | (20,616)
Total Fair Value | 1,628,610 | 1,533,889
Total Unrealized Losses | (56,354) | (27,326)
State and Municipal Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | 306,897 | 220,019
Less than 12 Months Unrealized Losses | (5,536) | (1,723)
12 Months or More Fair Value | 76,988 | 75,172
12 Months or More Unrealized Losses | (4,340) | (2,236)
Total Fair Value | 383,885 | 295,191
Total Unrealized Losses | (9,876) | (3,959)
US Government Corporations and Agencies Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | 261,185 | 184,046
Less than 12 Months Unrealized Losses | (4,942) | (1,006)
12 Months or More Fair Value | 109,734 | 155,983
12 Months or More Unrealized Losses | (2,286) | (1,756)
Total Fair Value | 370,919 | 340,029
Total Unrealized Losses | (7,228) | (2,762)
US Government Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | 247 | 249
Less than 12 Months Unrealized Losses | (4) | (3)
12 Months or More Fair Value | 0 | 0
12 Months or More Unrealized Losses | 0 | 0
Total Fair Value | 247 | 249
Total Unrealized Losses | $ (4) | $ (3)

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