Business Combinations (Narrative) (Details) shares in Millions		3 Months Ended	9 Months Ended
	Sep. 20, 2019 shares	Sep. 30, 2019 USD ($) loan	Sep. 30, 2019 USD ($) loan shares	Dec. 31, 2018 USD ($)	Jul. 13, 2018 USD ($)	Dec. 31, 2017 USD ($)
Business Acquisition [Line Items]
Goodwill		$ 861,291,000	$ 861,291,000	$ 790,972,000		$ 745,523,000
Purchased credit impaired loans book balance		269,204,000	269,204,000	308,401,000
Recorded investment without an allowance		47,178,000	47,178,000	$ 107,528,000
Sale of available-for-sale securities			143,300,000
MidSouth [Member]
Business Acquisition [Line Items]
Net assets acquired		124,642,000	124,642,000
Goodwill		69,207,000	$ 69,207,000
Shares issued | shares	5.0		5.0
Fixed conversion ratio			0.2952
Business combination transaction value on an average of share price			$ 193,800,000
Book balance of the loans acquired		686,000,000.0	686,000,000.0
Estimated uncollectible amount		17,200,000	17,200,000
Estimated fair value of uncollectible amount		667,200,000	667,200,000
Purchased credit impaired loans book balance		140,300,000	140,300,000
Recorded investment without an allowance		118,000,000.0	118,000,000.0
Sale of available-for-sale securities			143,300,000
Identifiable intangible assets		35,000,000	$ 35,000,000
Estimated useful life			15 years
Customer repurchase agreements		39,500,000	$ 39,500,000
Number of preferred debentures			3
Debt instrument maturity year			2037
Acquisition related costs		28,800,000	$ 28,810,000
Assets under management		$ 785,204,000	$ 785,204,000
MidSouth [Member] | FHLB
Business Acquisition [Line Items]
Number of FHLB advances | loan		2	2
FHLB advances amount		$ 27,500	$ 27,500
FHLB advances maturity period			30 days
FHLB advances fixed interest rate		2.16%	2.16%
Capital One [Member]
Business Acquisition [Line Items]
Net assets acquired		$ (187,218,000)	$ (187,218,000)
Goodwill		46,561,000	46,561,000
Identifiable intangible assets		27,562,000	$ 27,562,000
Assets under management					$ 4,000,000,000
Assets under management and administration					10,400,000,000
Cash for customer deposit liabilities assumed					$ 217,000,000
Trust And Asset Management Acquisition [Member]
Business Acquisition [Line Items]
Estimated useful life			17 years
Acquisition related costs			$ 5,700,000
Trust fee revenue			14,800,000
Related expense		$ 14,000,000.0	$ 14,000,000.0