Fair Value Measurements - Schedule of Financial Assets and Liabilities Measured at Fair Value (Detail) - USD ($)		Sep. 30, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held for sale		$ 412,194,000	$ 249,846,000
Derivative financial assets		7,074,000	6,504,000
Derivative financial liabilities		4,756,000	3,002,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		2,452,097,000	2,337,039,000
Available for sale equity securities		8,914,000	9,734,000
Loans held for sale		407,463,000	247,104,000
Derivative financial assets		7,074,000	6,504,000
Derivative financial liabilities		4,756,000	3,002,000
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		64,034,000	85,890,000
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		238,515,000	208,988,000
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		867,281,000	1,035,650,000
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		3,995,000	4,259,000
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		621,652,000	554,600,000
Asset-Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		279,707,000	271,970,000
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		5,069,000	5,917,000
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		16,544,000	8,362,000
Corporate Bonds [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale debt securities		355,300,000	161,403,000
Financial Services Industry [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		159,000	140,000
Other Equity Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Available for sale equity securities		4,790,000	4,640,000
Forward Sales Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		44,000	542,000
Interest Rate Lock Commitments [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		7,030,000	4,103,000
TBA Mortgage Backed Securities [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial liabilities		835,000	3,002,000
Interest Rate Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		0	1,859,000
Derivative financial liabilities		3,921,000
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]	3,965,000	4,954,000
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		6,673,000	6,822,000
Available for sale equity securities		8,914,000	9,734,000
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		6,673,000	6,822,000
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		159,000	140,000
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		4,790,000	4,640,000
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	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] | Interest Rate Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		0	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]	3,965,000	4,954,000
Significant Other Observable Inputs (Level 2) [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans held for sale		4,731,000	2,742,000
Derivative financial assets		44,000	2,401,000
Derivative financial liabilities		4,756,000	3,002,000
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		2,440,355,000	2,324,300,000
Available for sale equity securities		0	0
Loans held for sale		0	0
Derivative financial assets		44,000	2,401,000
Derivative financial liabilities		4,756,000	3,002,000
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		64,034,000	85,890,000
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		238,515,000	208,988,000
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		867,281,000	1,035,650,000
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		3,995,000	4,259,000
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		621,652,000	554,600,000
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		279,707,000	271,970,000
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		16,544,000	8,362,000
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		348,627,000	154,581,000
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		44,000	542,000
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		0	0
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		835,000	3,002,000
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		0	1,859,000
Derivative financial liabilities		3,921
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		407,463,000	247,104,000
Derivative financial assets		7,030,000	4,103,000
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		5,069,000	5,917,000
Available for sale equity securities		0	0
Loans held for sale		407,463,000	247,104,000
Derivative financial assets		7,030,000	4,103,000
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		5,069,000	5,917,000
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	0
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		7,030,000	4,103,000
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] | Interest Rate Contracts [Member] | Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Derivative financial assets		0	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.