Mergers and Acquisitions - Narrative (Details)		3 Months Ended
	Jul. 01, 2017 USD ($) branch shares	Sep. 30, 2018 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) branch	Dec. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)
Business Acquisition [Line Items]
Assets			$ 10,238,313,000	$ 9,829,981,000	$ 8,764,711,000
Deposits			8,357,769,000	7,921,075,000
Goodwill resulting from merger			$ 611,046,000	$ 611,046,000	$ 470,534,000
Metropolitan Bancgroup, Inc.
Business Acquisition [Line Items]
Cash settlement for stock options	$ 4,764,000
Transaction value	$ 219,461,000
Shares issued to common shareholders (in shares) | shares	4,883,182
Voting interest acquired (percent)	100.00%
Number of locations acquired | branch	8
Intangible assets, including goodwill	$ 147,478,000
Goodwill resulting from merger	140,512,000
Core deposit intangible	$ 6,966,000
Weighted average useful life (in years)	10 years
Scenario, Forecast | Brand Bank
Business Acquisition [Line Items]
Entity shares issued per acquiree share (in shares) | shares		32.87
Amount paid per acquiree share (in dollars per share) | $ / shares		$ 77.50
Cash settlement for stock options		$ 1,550
Brand
Business Acquisition [Line Items]
Number of locations | branch			13
Assets			$ 2,400,000,000
Loans			1,910,000,000
Deposits			$ 1,920,000,000