Acquisition Activity - Consideration (Details) - USD ($) $ in Thousands			9 Months Ended
	Mar. 23, 2018	Jul. 31, 2017	Sep. 30, 2018	Sep. 30, 2017	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Acquisition [Line Items]
Total equity consideration			$ 214,659	$ 211,043
Goodwill			1,220,903			$ 1,188,902	$ 726,856
Gibraltar Private Bank & Trust Company
Business Acquisition [Line Items]
Total equity consideration	$ 214,659
Cash	7
Total consideration paid	214,666
Fair value of net assets assumed including identifiable intangible assets	165,044
Goodwill	$ 49,622		$ 49,600
Gibraltar Private Bank & Trust Company | Common Stock
Business Acquisition [Line Items]
Common stock issued (shares)	2,787,773
Common stock issued	$ 214,659
Sabadell United
Business Acquisition [Line Items]
Total equity consideration		$ 211,043
Cash		809,159
Total consideration paid		1,020,202
Fair value of net assets assumed including identifiable intangible assets		579,157
Goodwill		$ 441,045			$ 441,000
Sabadell United | Common Stock
Business Acquisition [Line Items]
Common stock issued (shares)		2,610,304
Common stock issued		$ 211,043