Goodwill and Other Intangible Assets - Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	Jun. 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	96,720	106,071
Total goodwill and other intangible assets	942,407	948,722
Core Deposit Premium
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	105,984	105,984
Accumulated amortization	(21,425)	(16,659)
Other intangible assets, net	84,559	89,325
Books of Business Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	15,234	15,414
Accumulated amortization	(3,177)	(2,827)
Other intangible assets, net	12,057	12,587
Other intangibles, net
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	2,068	6,037
Accumulated amortization	(1,964)	(1,878)
Other intangible assets, net	$ 104	$ 4,159