Acquisition Activity - Additional Information (Details)				3 Months Ended				6 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 23, 2018 USD ($) $ / shares shares	Jul. 31, 2017 USD ($) bank shares	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Increase in goodwill due to adjustments								$ 34,870,000
Goodwill				$ 1,223,772,000	$ 1,223,772,000			1,223,772,000	$ 1,223,772,000		$ 1,188,902,000	$ 726,856,000
Amortization of intangible assets acquired				6,111,000			$ 1,651,000	11,213,000		$ 3,421,000
Gibraltar Private Bank & Trust Company
Business Acquisition [Line Items]
Loans acquired	$ 1,500,000,000	$ 1,465,278,000				$ 1,500,000,000
Interest-bearing deposits acquired		1,064,803,000
Cash consideration	$ 214,700,000	7,000
Increase in goodwill due to adjustments				10,000,000	52,500,000
Goodwill		$ 52,491,000		52,500,000	52,500,000			52,500,000	52,500,000
Gibraltar Private Bank & Trust Company | Common Stock
Business Acquisition [Line Items]
Number of shares issued per share acquired | $ / shares		$ 1.9749
Common stock issued (in dollars per share) | $ / shares		$ 77.00
Common stock issued (shares) | shares		2,787,773
Solomon Parks Title & Escrow
Business Acquisition [Line Items]
Cash consideration						3,300,000
Increase in goodwill due to adjustments						$ 3,400,000			3,400,000
Other Payments to Acquire Businesses								750,000
Sabadell United
Business Acquisition [Line Items]
Loans acquired			$ 4,030,777,000
Interest-bearing deposits acquired			4,382,780,000
Cash consideration			809,159,000
Increase in goodwill due to adjustments				22,400,000				21,000,000
Goodwill			$ 441,045,000	$ 441,000,000	$ 441,000,000			$ 441,000,000	$ 441,000,000
Number of offices acquired | bank			25
Percentage of voting interests acquired			100.00%
Sabadell United | Fair Value Adjustments
Business Acquisition [Line Items]
amortization of premiums and accretion of discounts on acquired loans, securities, and deposits							12,300,000			17,400,000
Elimination of amortization of premiums and accretion of discounts on previously acquired loans, securities, FDIC indemnification asset, and deposits							933,300			1,300,000
Allowance for loan and lease losses acquired							3,900,000			5,500,000
Amortization of intangible assets acquired							$ 3,600,000			$ 5,100,000
Sabadell United | Common Stock
Business Acquisition [Line Items]
Common stock issued (shares) | shares			2,610,304