Acquisition Activity - Additional Information (Details)				3 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 23, 2018 USD ($) $ / shares shares	Jul. 31, 2017 USD ($) bank shares	Mar. 31, 2018 USD ($)	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Increase in goodwill due to adjustments				$ 47,267,000
Amortization of intangible assets acquired				5,102,000		$ 1,770,000
Goodwill	$ 1,236,169,000			1,236,169,000	$ 1,236,169,000		$ 1,188,902,000	$ 726,856,000
Gibraltar Private Bank & Trust Company
Business Acquisition [Line Items]
Loans acquired	1,500,000,000	$ 1,465,319,000		1,500,000,000	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	42,500,000
Goodwill		$ 42,530,000
Gibraltar Private Bank & Trust Company | Common Stock
Business Acquisition [Line Items]
Common stock issued (in dollars per share) | $ / shares		$ 77.00
Number of shares issued per share acquired | $ / shares		$ 1.9749
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
Sabadell United
Business Acquisition [Line Items]
Loans acquired			$ 4,025,946,000
Interest-bearing deposits acquired			$ 4,382,780,000
Number of offices acquired | bank			25
Cash consideration			$ 809,159,000
Percentage of voting interests acquired			100.00%
Increase in goodwill due to adjustments				1,400,000
Goodwill	$ 463,400,000		$ 463,403,000	$ 463,400,000	$ 463,400,000
Sabadell United | Fair Value Adjustments
Business Acquisition [Line Items]
amortization of premiums and accretion of discounts on acquired loans, securities, and deposits						5,100,000
Elimination of amortization of premiums and accretion of discounts on previously acquired loans, securities, FDIC indemnification asset, and deposits						318,500
Allowance for loan and lease losses acquired						1,600,000
Amortization of intangible assets acquired						$ 1,500,000
Sabadell United | Common Stock
Business Acquisition [Line Items]
Common stock issued (shares) | shares			2,610,304