Acquisitions and Divestitures - Narrative (Details) $ in Thousands				3 Months Ended	9 Months Ended
	Mar. 23, 2018 USD ($) branch	Nov. 30, 2017 USD ($) branch shares	Apr. 03, 2017 USD ($)	Jun. 30, 2018 USD ($) shares	Sep. 30, 2018 USD ($) branch	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Goodwill					$ 1,409,822	$ 1,386,853	$ 236,335	$ 191,371
Number of branches divested | branch					2
Deposits disposed of						$ 22,600
Portion of Sub-prime Auto Loan Portfolios
Business Acquisition [Line Items]
Loans disposed of				$ 120,000	$ 120,000
Apex Bank
Business Acquisition [Line Items]
Number of branches divested | branch	2
Deposits disposed of	$ 30,000
Loans disposed of	$ 2,000
Capital Bank Financial Corporation
Business Acquisition [Line Items]
Business acquisition, equity interest issued or issuable, number of shares (in shares) | shares		92,042,232
Payments to acquire businesses		$ 423,600		$ 46,000	469,609
Business combination, consideration transferred		$ 2,200,000			2,216,333
Business acquisition, number of shares canceled (in shares) | shares				2,373,220
Number of bank branches | branch		178
Assets acquired		$ 9,900,000			9,857,979
Loans		7,300,000			7,276,544
Securities available-for-sale		1,200,000			1,193,393
Deposits		$ 8,100,000			8,140,102
Goodwill					1,169,874
Loans held-for-sale					124,918
Trading securities					0
Securities sold under agreements to repurchase					$ 26,664
Coastal Securities, Inc
Business Acquisition [Line Items]
Payments to acquire businesses			$ 131,000
Assets acquired			418,000
Goodwill			45,000
Loans held-for-sale			236,000
Trading securities			139,000
Securities sold under agreements to repurchase			202,000
Fixed income payables			$ 96,000