Acquisitions - Narrative (Details) $ in Thousands					3 Months Ended	6 Months Ended
	May 18, 2018 USD ($) shares	Feb. 23, 2018 USD ($) shares	Jul. 07, 2017 USD ($) building shares	Feb. 28, 2017 USD ($) building shares	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Assets acquired						$ 11,301,920	$ 9,810,739
Held to maturity						7,477,697	6,391,464
Deposits acquired						9,489,144	8,146,909
Goodwill resulting from acquisition						391,668	236,615
Nonaccrual						69,376	$ 62,581
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Purchase price	$ 189,905
Purchase price in cash	$ 5,451
Purchase price paid by delivery of shares of common stock (in shares) | shares	3,350,664
Outstanding debt extinguished	$ 3,900
Trust preferred securities assumed	12,077
Total assets acquired	1,120,000
Goodwill resulting from acquisition	121,400
Transaction costs	6,000
Pre-tax merger related costs						1,100
Nonaccrual						$ 7,600
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Purchase price		$ 61,400
Purchase price in cash		$ 7,800
Purchase price paid by delivery of shares of common stock (in shares) | shares		1,000,843
Goodwill resulting from acquisition		$ 33,700
Citywide Banks
Business Acquisition [Line Items]
Purchase price			$ 211,200
Purchase price in cash			$ 58,600
Purchase price paid by delivery of shares of common stock (in shares) | shares			3,216,161
Assets acquired			$ 1,490,000
Held to maturity			985,400
Deposits acquired			1,210,000
Goodwill resulting from acquisition			$ 95,200
Number of bank buildings sold (building) | building			1
Fair value of buildings sold			$ 1,400
Founders Bancorp
Business Acquisition [Line Items]
Purchase price				$ 31,000
Purchase price in cash				$ 8,400
Purchase price paid by delivery of shares of common stock (in shares) | shares				455,877
Assets acquired				$ 213,900
Deposits acquired				181,500
Goodwill resulting from acquisition				$ 13,800
Number of bank buildings sold (building) | building				1
Fair value of buildings sold					$ 576
Loans acquired				$ 96,400
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Held to maturity	681,100
Deposits acquired	893,800
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Outstanding debt extinguished	5,900
Assets acquired		427,100
Held to maturity		324,500
Deposits acquired		$ 357,300
Stock Appreciation Rights (SARs) | First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Payment to holders of FBLB's stock appreciation rights	13,300
Payment for stock appreciation rights	13,300
Trust Preferred Securities Subject to Mandatory Redemption | First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Trust preferred securities assumed	$ 8,200