Goodwill and Other Intangible Assets - Carrying Amount of Goodwill (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Changes In Carrying Amount of Goodwill
Beginning balance	$ 939,683	$ 932,928
Measurement period adjustments to goodwill from Brand acquisition		6,755
Additions to goodwill and other adjustments	0
Ending balance	939,683	939,683
Community Banks
Changes In Carrying Amount of Goodwill
Beginning balance	936,916	930,161
Measurement period adjustments to goodwill from Brand acquisition		6,755
Additions to goodwill and other adjustments	0
Ending balance	936,916	936,916
Insurance
Changes In Carrying Amount of Goodwill
Beginning balance	2,767	2,767
Measurement period adjustments to goodwill from Brand acquisition		0
Additions to goodwill and other adjustments	0
Ending balance	$ 2,767	$ 2,767

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Section 45
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Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-SubTopic 20
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Amount of increase (decrease) from adjustments after acquisition date under purchase accounting of an asset representing the future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 10
-Section 25
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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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