Acquisitions (Details) $ in Thousands				3 Months Ended
	Feb. 23, 2018 USD ($) shares	Jul. 07, 2017 USD ($) building shares	Feb. 28, 2017 USD ($) building shares	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Assets						$ 10,055,863	$ 9,810,739
Held to maturity						6,746,015	6,391,464
Deposits						8,541,540	$ 8,146,909
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
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		$ 1,490,000
Held to maturity		985,400
Deposits		$ 1,210,000
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			$ 213,900
Deposits			$ 181,500
Number of bank buildings sold (building) | building			1
Fair value of buildings sold					$ 576
Loans			$ 96,400
First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Assets						971,500
Held to maturity						704,900
Deposits						$ 869,300
Signature Bancshares, Inc.
Business Acquisition [Line Items]
Assets	$ 427,100
Held to maturity	324,500
Deposits	$ 357,300
Scenario, Forecast [Member] | First Bank Lubbock Bancshares, Inc.
Business Acquisition [Line Items]
Purchase price				$ 185,600