Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($) $ in Thousands		Jun. 30, 2020	Dec. 31, 2019
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held for sale		$ 625,984	$ 387,514
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		2,799,941	2,437,296
Available for sale equity securities		9,875	8,894
Loans held for sale		611,277	384,375
Derivative financial assets		29,578	4,527
Derivative financial liabilities		14,307	3,065
U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		67,303	58,676
State and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		520,880	272,362
Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		848,231	836,534
Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Non-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		62,965	3,833
Commercial Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		622,785	614,973
Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		277,721	276,139
Trust Preferred Collateralized Debt Obligations [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		12,837	4,703
Single Issue Trust Preferred Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		16,191	16,774
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		371,028	353,302
Financial Services Industry [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		103	154
Other Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		5,989	4,769
Forward Sales Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		1,745	4,518
Interest Rate Lock Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		27,833
TBA Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities		4,224	671
Derivative financial liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities			2,394
Interest Rate Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets			9
Derivative financial liabilities		10,083
Equity Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities	[1]	3,783	3,971
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held for sale		0	0
Derivative financial assets		0	0
Derivative financial liabilities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		6,432	6,586
Available for sale equity securities		9,875	8,894
Loans held for sale		0	0
Derivative financial assets		0	0
Derivative financial liabilities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | State and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Non-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Commercial Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Trust Preferred Collateralized Debt Obligations [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Single Issue Trust Preferred Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		6,432	6,586
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Financial Services Industry [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		103	154
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Other Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		5,989	4,769
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Forward Sales Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Interest Rate Lock Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | TBA Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities		0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Derivative financial liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities			0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Interest Rate Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets			0
Derivative financial liabilities		0
Quoted Prices in Active Markets for Identical Assets (Level 1) [Member] | Equity Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities	[1]	3,783	3,971
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held for sale		60,187	3,139
Derivative financial assets		9,158	9
Derivative financial liabilities		14,307	3,065
Significant Other Observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		2,780,672	2,426,007
Available for sale equity securities		0	0
Loans held for sale		45,480	0
Derivative financial assets		9,158	9
Derivative financial liabilities		14,307	3,065
Significant Other Observable Inputs (Level 2) [Member] | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		67,303	58,676
Significant Other Observable Inputs (Level 2) [Member] | State and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		520,880	272,362
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		848,231	836,534
Significant Other Observable Inputs (Level 2) [Member] | Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Non-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		62,965	3,833
Significant Other Observable Inputs (Level 2) [Member] | Commercial Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		622,785	614,973
Significant Other Observable Inputs (Level 2) [Member] | Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		277,721	276,139
Significant Other Observable Inputs (Level 2) [Member] | Trust Preferred Collateralized Debt Obligations [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Other Observable Inputs (Level 2) [Member] | Single Issue Trust Preferred Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		16,191	16,774
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		364,596	346,716
Significant Other Observable Inputs (Level 2) [Member] | Financial Services Industry [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		0	0
Significant Other Observable Inputs (Level 2) [Member] | Other Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		0	0
Significant Other Observable Inputs (Level 2) [Member] | Forward Sales Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		1,745	0
Significant Other Observable Inputs (Level 2) [Member] | Interest Rate Lock Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		7,413
Significant Other Observable Inputs (Level 2) [Member] | TBA Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities		4,224	671
Significant Other Observable Inputs (Level 2) [Member] | Derivative financial liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities			2,394
Significant Other Observable Inputs (Level 2) [Member] | Interest Rate Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets			9
Derivative financial liabilities		10,083
Significant Other Observable Inputs (Level 2) [Member] | Equity Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities	[1]	0	0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held for sale		565,797	384,375
Derivative financial assets		20,420	4,518
Derivative financial liabilities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		12,837	4,703
Available for sale equity securities		0	0
Loans held for sale		565,797	384,375
Derivative financial assets		20,420	4,518
Derivative financial liabilities		0	0
Significant Unobservable Inputs (Level 3) [Member] | U.S. Treasury Securities and Obligations of U.S. Government Corporations and Agencies [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | State and Political Subdivisions [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Residential Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Non-Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Commercial Mortgage-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member] | Agency [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Trust Preferred Collateralized Debt Obligations [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		12,837	4,703
Significant Unobservable Inputs (Level 3) [Member] | Single Issue Trust Preferred Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Financial Services Industry [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Other Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Forward Sales Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		0	4,518
Significant Unobservable Inputs (Level 3) [Member] | Interest Rate Lock Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		20,420
Significant Unobservable Inputs (Level 3) [Member] | TBA Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities		0	0
Significant Unobservable Inputs (Level 3) [Member] | Derivative financial liabilities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities			0
Significant Unobservable Inputs (Level 3) [Member] | Interest Rate Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets			0
Derivative financial liabilities		0
Significant Unobservable Inputs (Level 3) [Member] | Equity Mutual Funds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities	[1]	$ 0	$ 0

[1] The equity mutual funds are within a rabbi trust for the payment of benefits under a deferred compensation plan for certain key officers of United and its subsidiaries.