Goodwill and Other Intangible Assets - Goodwill and Other Intangibles (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill	$ 926,648	$ 845,687
Finite-Lived Intangible Assets [Line Items]
Other intangible assets, net	101,149	91,334
Total goodwill and other intangible assets	1,027,797	937,021
Core Deposit Premium
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	124,334	105,984
Accumulated amortization	(33,918)	(26,177)
Other intangible assets, net	90,416	79,807
Books of Business Intangible
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	15,234	15,234
Accumulated amortization	(4,501)	(3,707)
Other intangible assets, net	$ 10,733	$ 11,527