Acquisition Activity - Assets Acquired and Liabilities Assumed (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Jun. 30, 2018	Mar. 23, 2018	Jan. 12, 2018	Dec. 31, 2017	Jul. 31, 2017	Dec. 31, 2016
Assets
Non-compete agreement				$ 156
Liabilities
Goodwill	$ 1,220,903				$ 1,188,902		$ 726,856
Gibraltar Private Bank & Trust Company
Assets
Cash and cash equivalents			$ 102,575
Investment securities			19,169
Equity securities			27,519
Loans			1,465,278
Core deposit intangible assets			18,529
Other assets			12,005
Total assets acquired			1,645,075
Liabilities
Deposit liabilities			1,064,803
Long-term borrowings			405,107
Deferred tax liability, net			5,960
Other liabilities			4,161
Total liabilities assumed			1,480,031
Goodwill	$ 49,600		$ 49,622
Sabadell United
Assets
Cash and cash equivalents						$ 318,819
Investment securities						964,123
Loans						4,030,777
Core deposit intangible assets						66,600
Deferred tax asset, net						44,480
Other assets						92,820
Total assets acquired						5,517,619
Liabilities
Deposit liabilities						4,382,780
Short-term borrowings						520,539
Other liabilities						35,143
Total liabilities assumed						4,938,462
Goodwill		$ 441,000				$ 441,045