Related Party Transactions (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Mar. 16, 2017	Mar. 16, 2012
Related Party Transaction [Line Items]
Securities purchased under agreements to resell	$ 144,094,000		$ 144,094,000		$ 178,914,000
Securities sold under agreements to repurchase	189,474,000		189,474,000		173,028,000
Preferred stock	229,475,000		229,475,000		229,475,000
Preferred stock dividends	3,286,000	$ 4,561,000	12,443,000	$ 14,808,000
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	142,344,000		142,344,000		178,914,000
Securities sold under agreements to repurchase	155,678,000		155,678,000		16,596,000
BBVA | BBVA Compass Bancshares, Inc.
Related Party Transaction [Line Items]
Revenue from related parties	4,400,000	10,200,000	12,900,000	21,800,000
Fees on agreements	13,800,000	9,400,000	39,100,000	26,000,000
Derivatives designated as hedging instrument | BBVA
Related Party Transaction [Line Items]
Derivative, notional amount	3,200,000,000		3,200,000,000		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	(52,206,000)		(52,206,000)		(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			11,400,000	13,800,000
Cash flow hedges | Derivatives designated as hedging instrument | BBVA
Related Party Transaction [Line Items]
Related party transaction, amount of transaction	(87,000)		(87,000)		102,000
Fair value hedges | BBVA
Related Party Transaction [Line Items]
Related party transaction, amount of transaction	(837,000)		(837,000)		(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	8,000	24,000	67,000	50,000
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