Assets and Liabilities Measured at Fair Value - Assets and liabilities measured at fair value on nonrecurring basis (Details 2) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Level 1
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans
Level 2
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans
Level 3
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans	3,980,932	3,957,669	3,825,482
Total
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans	3,980,932	3,957,669	3,825,482
Nonrecurring | Level 1
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans
Foreclosed properties
Total assets
Nonrecurring | Level 2
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans
Foreclosed properties
Total assets
Nonrecurring | Level 3
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans	170,767	165,751	176,632
Foreclosed properties	14,716	14,788	27,675
Total assets	185,483	180,539	204,307
Nonrecurring | Total
Presentation of United's assets and liabilities measured at fair value on nonrecurring basis
Loans	170,767	165,751	176,632
Foreclosed properties	14,716	14,788	27,675
Total assets	$ 185,483	$ 180,539	$ 204,307