Deposits - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Deposits [Abstract]
FDIC insurance limit	$ 250,000
Time deposits (excluding brokered time deposits) that met or exceeded FDIC insurance limit	262,000,000	$ 205,000,000
Brokered deposits, certificates of deposit	544,000,000	133,000,000
Daily average balance of brokered deposits	$ 347,000,000	$ 109,000,000
Description of fair value hedging instrument	United has certain market-linked brokered deposits that are considered hybrid instruments that contain embedded derivatives that have been bifurcated from the host contract leaving host instruments paying a rate of 90 day LIBOR minus a spread that, at times, has resulted in a negative yield.