Commitments and Contingencies - Commitments and Letters of Credit (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Other Commitments [Line Items]
Commitments and letters of credit	$ 1,503,621	$ 1,501,346
Commitments to extend credit
Other Commitments [Line Items]
Commitments and letters of credit	1,430,863	1,420,428
Standby letters of credit
Other Commitments [Line Items]
Commitments and letters of credit	$ 72,758	$ 80,918

Maximum amount of loss due to credit risk that, based on the gross fair value of the financial instrument, the entity would incur if parties to the financial instruments that make up the concentration failed completely to perform according to the terms of the contracts and the collateral or other security, if any, for the amount due proved to be of no value to the entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 21
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109250915&loc=d3e13537-108611

Name:	us-gaap_ConcentrationRiskCreditRiskFinancialInstrumentMaximumExposure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration