Acquisitions - Narrative (Details) $ / shares in Units, $ in Thousands				3 Months Ended	9 Months Ended		12 Months Ended
	Jul. 14, 2017 USD ($) bank_branch shares	May 19, 2017 USD ($) bank_branch shares	Nov. 21, 2016 USD ($) $ / shares shares	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Less: Common stock issued						$ 321,431
IAB
Business Acquisition [Line Items]
Shares purchased (in shares) | shares			495,112
Percentage of interest acquired			12.10%	12.10%		12.10%
Value of acquired shares			$ 19,800
Purchase price per share (usd per share) | $ / shares			$ 40.00
Number of banking centers acquired | bank_branch	16
Stock issued as a part of acquisition (in shares) | shares	6,000,000
Less: Common stock issued	$ 238,800
Total purchase price	258,600
Fair value of equity interest after adjustment				$ 19,800
Remeasurement loss				$ 50
Other intangible assets	17,403
Operating revenue							$ 21,400
IAB | Core deposit intangibles acquired
Business Acquisition [Line Items]
Other intangible assets	$ 13,638
Acquired intangible asset, expected useful life	10 years
IAB | Other intangibles acquired
Business Acquisition [Line Items]
Other intangible assets	$ 3,765
Acquired intangible asset, expected useful life	2 years
IAB | Common Stock
Business Acquisition [Line Items]
Number of shares of common stock	1.653
Arlington Bank
Business Acquisition [Line Items]
Percentage of interest acquired		100.00%
Number of banking centers acquired | bank_branch		3
Stock issued as a part of acquisition (in shares) | shares		2,100,000
Less: Common stock issued		$ 82,600
Other intangible assets		4,526
Operating revenue							9,000
Arlington Bank | Core deposit intangibles acquired
Business Acquisition [Line Items]
Other intangible assets		$ 4,526
Acquired intangible asset, expected useful life		10 years
Arlington Bank | Common Stock
Business Acquisition [Line Items]
Number of shares of common stock		2.7245
Arlington Bank and IAB | Expenses directly attributable to acquisitions
Business Acquisition [Line Items]
Non-recurring expenses							$ 15,400