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Allowance for Credit Losses and Credit Quality of Loans, Allowance for Loan Losses and Recorded Investment in Loans (Details) - USD ($) $ in Thousands | Dec. 31, 2019 | Mar. 31, 2019 | Dec. 31, 2018
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Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Allowance for loan losses | $ 72,965 | [1] | $ 71,405 | $ 72,505
Allowance for loans individually evaluated for impairment | 0 |  |  |
Allowance for loans collectively evaluated for impairment | 72,965 |  |  |
Ending balance of loans | 7,136,098 |  |  |
Commercial Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Allowance for loan losses | 34,525 |  | 32,159 | 32,759
Allowance for loans individually evaluated for impairment | 0 |  |  |
Allowance for loans collectively evaluated for impairment | 34,525 |  |  |
Ending balance of loans | 3,444,266 |  |  |
Consumer Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Allowance for loan losses | 35,647 |  | 36,804 | 37,178
Allowance for loans individually evaluated for impairment | 0 |  |  |
Allowance for loans collectively evaluated for impairment | 35,647 |  |  |
Ending balance of loans | 2,246,676 |  |  |
Residential Real Estate [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Allowance for loan losses | 2,793 |  | $ 2,442 | $ 2,568
Allowance for loans individually evaluated for impairment | 0 |  |  |
Allowance for loans collectively evaluated for impairment | 2,793 |  |  |
Ending balance of loans | 1,445,156 |  |  |
Originated Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans individually evaluated for impairment | 18,253 |  |  |
Ending balance of loans collectively evaluated for impairment | 6,852,967 |  |  |
Originated Loans [Member] | Commercial Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans individually evaluated for impairment | 3,488 |  |  |
Ending balance of loans collectively evaluated for impairment | 3,325,512 |  |  |
Originated Loans [Member] | Consumer Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans individually evaluated for impairment | 7,044 |  |  |
Ending balance of loans collectively evaluated for impairment | 2,215,899 |  |  |
Originated Loans [Member] | Residential Real Estate [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans individually evaluated for impairment | 7,721 |  |  |
Ending balance of loans collectively evaluated for impairment | 1,311,556 |  |  |
Acquired Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans collectively evaluated for impairment | 264,878 |  |  |
Acquired Loans [Member] | Commercial Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans collectively evaluated for impairment | 115,266 |  |  |
Acquired Loans [Member] | Consumer Loans [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans collectively evaluated for impairment | 23,733 |  |  |
Acquired Loans [Member] | Residential Real Estate [Member] |  |  |  |
Allowance for loan losses and recorded investment by portfolio segment [Abstract] |  |  |  |
Ending balance of loans collectively evaluated for impairment | $ 125,879 |  |  |
[1] | Beginning January 1, 2020, calculation is based on current expected loss methodology. Prior to January 1, 2020, calculation was based on incurred loss methodology.

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- Definition The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524 + Details
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- Definition Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524 + Details
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- Definition Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.7(d)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (g) -URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 30 -Section 50 -Paragraph 2 -Subparagraph (b)(1)(ii) -URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 11B -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section S99 -Paragraph 4 -Subparagraph (SAB Topic 6.L.1) -URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707 + Details
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