Business Combinations - Additional Information (Details) $ / shares in Units, $ in Thousands		3 Months Ended	12 Months Ended
	Apr. 07, 2017 USD ($) Office	Jun. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 14, 2016 $ / shares	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Goodwill			$ 379,627	$ 366,156		$ 366,156
Intangible asset estimated useful life			20 years
Reliance [Member]
Business Acquisition [Line Items]
Number of offices | Office	7
Business acquisition agreement date	Nov. 14, 2016
Business acquisition, share price | $ / shares					$ 22.00
Total consideration paid	$ 24,800
Fair value measurement period from acquisition date, maximum	1 year
Goodwill	$ 13,471		$ 13,500
Intangible asset estimated useful life	10 years
Reliance [Member] | Other Noninterest Expense [Member]
Business Acquisition [Line Items]
Merger transaction expense		$ 3,200
Reliance [Member] | Other Noninterest Expense [Member] | Change In Control Expense [Member]
Business Acquisition [Line Items]
Merger transaction expense		1,300
Reliance [Member] | Other Noninterest Expense [Member] | Professional Fees and Contract Termination and Other Expenses [Member]
Business Acquisition [Line Items]
Merger transaction expense		$ 1,900
Reliance [Member] | Reliance Common Stock [Member]
Business Acquisition [Line Items]
Business acquisition, payment for equity	$ 23,700
Reliance [Member] | Reliance Preferred Stock [Member]
Business Acquisition [Line Items]
Business acquisition, payment for equity	$ 1,100