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Securities - Fair Value and Age of Gross Unrealized Losses of Debt Securities (Details) $ in Thousands | Mar. 31, 2018 USD ($) security | Dec. 31, 2017 USD ($) security
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Schedule of Available-for-sale Securities [Line Items] |  |
Temporarily Impaired Debt Securities, Less Than 12 Months, Number of Securities | security | 56 | 36
Temporarily Impaired Debt Securities, Less Than 12 Months, Fair Value | $ 430,791 | $ 271,852
Temporarily Impaired Debt Securities, Less Than 12 Months, Unrealized Losses | $ (6,712) | $ (1,373)
Temporarily Impaired Debt Securities, 12 Months or More, Number of Securities | security | 15 | 15
Temporarily Impaired Debt Securities, 12 Months or More, Fair Value | $ 108,907 | $ 113,210
Temporarily Impaired Debt Securities, 12 Months or More, Unrealized Losses | $ (4,155) | $ (2,594)
Temporarily Impaired Debt Securities, Number of Securities | security | 71 | 51
Temporarily Impaired Debt Securities, Fair Value, Total | $ 539,698 | $ 385,062
Temporarily Impaired Debt Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (10,867) | $ (3,967)
U.S. Treasury securities |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Temporarily Impaired Debt Securities, Less Than 12 Months, Number of Securities | security | 2 | 3
Temporarily Impaired Debt Securities, Less Than 12 Months, Fair Value | $ 14,661 | $ 19,789
Temporarily Impaired Debt Securities, Less Than 12 Months, Unrealized Losses | $ (290) | $ (154)
Temporarily Impaired Debt Securities, 12 Months or More, Number of Securities | security | 0 | 0
Temporarily Impaired Debt Securities, 12 Months or More, Fair Value | $ 0 | $ 0
Temporarily Impaired Debt Securities, 12 Months or More, Unrealized Losses | $ 0 | $ 0
Temporarily Impaired Debt Securities, Number of Securities | security | 2 | 3
Temporarily Impaired Debt Securities, Fair Value, Total | $ 14,661 | $ 19,789
Temporarily Impaired Debt Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (290) | $ (154)
Obligations of U.S. government corporations and agencies |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Temporarily Impaired Debt Securities, Less Than 12 Months, Number of Securities | security | 16 | 9
Temporarily Impaired Debt Securities, Less Than 12 Months, Fair Value | $ 130,823 | $ 63,635
Temporarily Impaired Debt Securities, Less Than 12 Months, Unrealized Losses | $ (1,274) | $ (144)
Temporarily Impaired Debt Securities, 12 Months or More, Number of Securities | security | 1 | 1
Temporarily Impaired Debt Securities, 12 Months or More, Fair Value | $ 10,000 | $ 10,017
Temporarily Impaired Debt Securities, 12 Months or More, Unrealized Losses | $ (43) | $ (49)
Temporarily Impaired Debt Securities, Number of Securities | security | 17 | 10
Temporarily Impaired Debt Securities, Fair Value, Total | $ 140,823 | $ 73,652
Temporarily Impaired Debt Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (1,317) | $ (193)
Collateralized mortgage obligations of U.S. government corporations and agencies |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Temporarily Impaired Debt Securities, Less Than 12 Months, Number of Securities | security | 8 | 7
Temporarily Impaired Debt Securities, Less Than 12 Months, Fair Value | $ 55,695 | $ 47,465
Temporarily Impaired Debt Securities, Less Than 12 Months, Unrealized Losses | $ (800) | $ (248)
Temporarily Impaired Debt Securities, 12 Months or More, Number of Securities | security | 7 | 7
Temporarily Impaired Debt Securities, 12 Months or More, Fair Value | $ 43,184 | $ 45,809
Temporarily Impaired Debt Securities, 12 Months or More, Unrealized Losses | $ (1,683) | $ (1,073)
Temporarily Impaired Debt Securities, Number of Securities | security | 15 | 14
Temporarily Impaired Debt Securities, Fair Value, Total | $ 98,879 | $ 93,274
Temporarily Impaired Debt Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (2,483) | $ (1,321)
Residential mortgage-backed securities of U.S. government corporations and agencies |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Temporarily Impaired Debt Securities, Less Than 12 Months, Number of Securities | security | 3 | 1
Temporarily Impaired Debt Securities, Less Than 12 Months, Fair Value | $ 4,861 | $ 2,333
Temporarily Impaired Debt Securities, Less Than 12 Months, Unrealized Losses | $ (106) | $ (10)
Temporarily Impaired Debt Securities, 12 Months or More, Number of Securities | security | 2 | 2
Temporarily Impaired Debt Securities, 12 Months or More, Fair Value | $ 7,976 | $ 8,638
Temporarily Impaired Debt Securities, 12 Months or More, Unrealized Losses | $ (354) | $ (203)
Temporarily Impaired Debt Securities, Number of Securities | security | 5 | 3
Temporarily Impaired Debt Securities, Fair Value, Total | $ 12,837 | $ 10,971
Temporarily Impaired Debt Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (460) | $ (213)
Commercial mortgage-backed securities of U.S. government corporations and agencies |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Temporarily Impaired Debt Securities, Less Than 12 Months, Number of Securities | security | 22 | 14
Temporarily Impaired Debt Securities, Less Than 12 Months, Fair Value | $ 201,051 | $ 128,300
Temporarily Impaired Debt Securities, Less Than 12 Months, Unrealized Losses | $ (4,170) | $ (775)
Temporarily Impaired Debt Securities, 12 Months or More, Number of Securities | security | 5 | 5
Temporarily Impaired Debt Securities, 12 Months or More, Fair Value | $ 47,747 | $ 48,746
Temporarily Impaired Debt Securities, 12 Months or More, Unrealized Losses | $ (2,075) | $ (1,269)
Temporarily Impaired Debt Securities, Number of Securities | security | 27 | 19
Temporarily Impaired Debt Securities, Fair Value, Total | $ 248,798 | $ 177,046
Temporarily Impaired Debt Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (6,245) | $ (2,044)
Obligations of states and political subdivisions |  |
Schedule of Available-for-sale Securities [Line Items] |  |
Temporarily Impaired Debt Securities, Less Than 12 Months, Number of Securities | security | 5 | 2
Temporarily Impaired Debt Securities, Less Than 12 Months, Fair Value | $ 23,700 | $ 10,330
Temporarily Impaired Debt Securities, Less Than 12 Months, Unrealized Losses | $ (72) | $ (42)
Temporarily Impaired Debt Securities, 12 Months or More, Number of Securities | security | 0 | 0
Temporarily Impaired Debt Securities, 12 Months or More, Fair Value | $ 0 | $ 0
Temporarily Impaired Debt Securities, 12 Months or More, Unrealized Losses | $ 0 | $ 0
Temporarily Impaired Debt Securities, Number of Securities | security | 5 | 2
Temporarily Impaired Debt Securities, Fair Value, Total | $ 23,700 | $ 10,330
Temporarily Impaired Debt Securities, Continuous Unrealized Loss Position, Accumulated Loss | $ (72) | $ (42)

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