Goodwill and Other Intangible Assets - Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 845,687	$ 842,651
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	93,975	106,071
Total goodwill and other intangible assets	939,662	948,722
Core Deposit Premium
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	105,984	105,984
Accumulated amortization	(23,802)	(16,659)
Other intangible assets, net	82,182	89,325
Books of Business Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	15,234	15,414
Accumulated amortization	(3,441)	(2,827)
Other intangible assets, net	11,793	12,587
Other intangibles, net
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	2,068	6,037
Accumulated amortization	(2,068)	(1,878)
Other intangible assets, net	$ 0	$ 4,159