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Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017 | Mar. 31, 2017
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | $ 961,589 | $ 674,104 | $ 2,496
Gross Unrealized Gains | 5,263 | 7,872 |
Gross Unrealized Losses | (18,487) | (10,488) |
Fair Value | 948,365 | 671,488 |
Trust preferred securities |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 12,429 | 12,442 |
Gross Unrealized Gains | 0 | 0 |
Gross Unrealized Losses | (2,384) | (3,054) |
Fair Value | 10,045 | 9,388 |
Other debt securities |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 23,994 | 17,106 |
Gross Unrealized Gains | 152 | 260 |
Gross Unrealized Losses | (206) | (76) |
Fair Value | 23,940 | 17,290 |
Obligations of other U.S. Government agencies and corporations |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 3,549 | 3,554 |
Gross Unrealized Gains | 27 | 40 |
Gross Unrealized Losses | (48) | (30) |
Fair Value | 3,528 | 3,564 |
Obligations of states and political subdivisions |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 222,814 | 228,589 |
Gross Unrealized Gains | 4,292 | 6,161 |
Gross Unrealized Losses | (764) | (269) |
Fair Value | 226,342 | 234,481 |
Government agency mortgage backed securities | Residential mortgage backed securities: |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 416,954 | 196,121 |
Gross Unrealized Gains | 444 | 888 |
Gross Unrealized Losses | (7,928) | (3,059) |
Fair Value | 409,470 | 193,950 |
Government agency mortgage backed securities | Commercial mortgage backed securities: |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 27,636 | 31,015 |
Gross Unrealized Gains | 306 | 389 |
Gross Unrealized Losses | (440) | (234) |
Fair Value | 27,502 | 31,170 |
Government agency collateralized mortgage obligations | Residential mortgage backed securities: |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 244,213 | 180,258 |
Gross Unrealized Gains | 42 | 133 |
Gross Unrealized Losses | (6,589) | (3,752) |
Fair Value | 237,666 | 176,639 |
Government agency collateralized mortgage obligations | Commercial mortgage backed securities: |  |  |
Amortized cost and fair value of securities available for sale |  |  |
Amortized Cost | 10,000 | 5,019 |
Gross Unrealized Gains | 0 | 1 |
Gross Unrealized Losses | (128) | (14) |
Fair Value | $ 9,872 | $ 5,006 |

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- Definition This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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