Related Party Transactions (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019	Mar. 16, 2017	Mar. 16, 2012
Related Party Transaction [Line Items]
Securities purchased under agreements to resell	$ 167,469,000		$ 167,469,000		$ 178,914,000
Securities sold under agreements to repurchase	249,481,000		249,481,000		173,028,000
Preferred stock	229,475,000		229,475,000		229,475,000
Preferred stock dividends	5,002,000	$ 5,762,000	9,157,000	$ 10,247,000
Transfer of loans to loans held for sale			0	1,196,883,000
BBVA
Related Party Transaction [Line Items]
Securities purchased under agreements to resell	142,886,000		142,886,000		178,914,000
Securities sold under agreements to repurchase	29,731,000		29,731,000		16,596,000
BBVA | BBVA Compass Bancshares, Inc.
Related Party Transaction [Line Items]
Revenue from related parties	4,400,000	7,500,000	8,500,000	11,600,000
Fees on agreements	15,400,000	8,000,000	25,300,000	16,500,000
Derivatives designated as hedging instrument | BBVA
Related Party Transaction [Line Items]
Derivative, notional amount	3,600,000,000		3,600,000,000		3,400,000,000
Free-standing derivatives not designated as hedging instrument | Free-standing derivative instruments – risk management and other purposes | BBVA
Related Party Transaction [Line Items]
Related party transaction, amount of transaction	(58,352,000)		(58,352,000)		(9,688,000)
Preferred Stock | Series A Preferred Stock | BBVA
Related Party Transaction [Line Items]
Preferred stock	229,000,000		229,000,000		229,000,000
Preferred stock dividends			8,100,000	9,200,000
Cash flow hedges | Derivatives designated as hedging instrument | BBVA
Related Party Transaction [Line Items]
Related party transaction, amount of transaction	(303,000)		(303,000)		102,000
Fair value hedges | BBVA
Related Party Transaction [Line Items]
Related party transaction, amount of transaction	(841,000)		(841,000)		(354,000)
BSI | Revolving Credit Facility | Line of Credit | BBVA Compass Bancshares, Inc.
Related Party Transaction [Line Items]
Due from related parties	0		0		$ 0	$ 450,000,000	$ 420,000,000
BSI | Uncommitted Demand Facility | Revolving Credit Facility | Line of Credit | BBVA Compass Bancshares, Inc.
Related Party Transaction [Line Items]
Due from related parties						$ 1,000,000,000
BSI | Revolving Note and Cash Subordinated Agreement | BBVA Compass Bancshares, Inc.
Related Party Transaction [Line Items]
Interest expense	4,000	0	59,000	25,000
Loans and Loans Held for Sale Excluding Loans Originated for Sale in Secondary Market
Related Party Transaction [Line Items]
Transfer of loans to loans held for sale	$ 0	$ 0	$ 0	1,196,883,000
Gain on sale of loans				$ 778,000