Goodwill and Other Intangible Assets - Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 845,687	$ 845,687
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	88,694	91,334
Total goodwill and other intangible assets	934,381	937,021
Core Deposit Premium
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	105,984	105,984
Accumulated amortization	(28,552)	(26,177)
Other intangible assets, net	77,432	79,807
Books of Business Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	15,234	15,234
Accumulated amortization	(3,972)	(3,707)
Other intangible assets, net	$ 11,262	$ 11,527