Other Intangibles - Schedule of Core Deposit and Other Intangibles (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2017	Dec. 31, 2018
Finite-Lived Intangible Assets [Line Items]
Gross carrying amount	$ 85,869		$ 85,869
Accumulated amortization	(65,844)		(61,440)
Total other intangibles	36,552		$ 24,429
Core deposit intangibles acquired
Finite-Lived Intangible Assets [Line Items]
Core deposit intangibles acquired	$ 16,527	$ 0