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Securities (Summary of Gross Unrealized Losses and Fair Value of the Investments with Unrealized Losses) (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | $ 1,718,571 | $ 1,220,992
Less than 12 Months Unrealized Losses | (40,068) | (9,442)
12 Months or More Fair Value | 1,051,352 | 948,366
12 Months or More Unrealized Losses | (50,463) | (24,608)
Total Fair Value | 2,769,923 | 2,169,358
Total Unrealized Losses | (90,531) | (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 | 1,084,529 | 816,678
Less than 12 Months Unrealized Losses | (26,966) | (6,710)
12 Months or More Fair Value | 834,985 | 717,211
12 Months or More Unrealized Losses | (42,233) | (20,616)
Total Fair Value | 1,919,514 | 1,533,889
Total Unrealized Losses | (69,199) | (27,326)
State and Municipal Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | 333,523 | 220,019
Less than 12 Months Unrealized Losses | (6,833) | (1,723)
12 Months or More Fair Value | 96,018 | 75,172
12 Months or More Unrealized Losses | (5,408) | (2,236)
Total Fair Value | 429,541 | 295,191
Total Unrealized Losses | (12,241) | (3,959)
US Government Corporations and Agencies Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | 300,519 | 184,046
Less than 12 Months Unrealized Losses | (6,269) | (1,006)
12 Months or More Fair Value | 120,102 | 155,983
12 Months or More Unrealized Losses | (2,818) | (1,756)
Total Fair Value | 420,621 | 340,029
Total Unrealized Losses | (9,087) | (2,762)
US Government Securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Less than 12 Months Fair Value | 0 | 249
Less than 12 Months Unrealized Losses | 0 | (3)
12 Months or More Fair Value | 247 | 0
12 Months or More Unrealized Losses | (4) | 0
Total Fair Value | 247 | 249
Total Unrealized Losses | $ (4) | $ (3)

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