Acquisitions (Narrative) (Details) $ in Thousands			12 Months Ended
	Apr. 28, 2017 USD ($) entity	Mar. 10, 2017 USD ($) entity	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Apr. 29, 2017 entity
Business Acquisition [Line Items]
Short-term borrowings			$ 1,703,890	$ 1,225,406
Long-term borrowings			305,513	436,280
Goodwill			745,523	621,193
Revenue			792,312	659,116	$ 625,174
Professional Fees			40,235	29,561	40,198
Other expense			82,636	72,564	72,203
Loan expected to be uncollectible			217,308	229,418	$ 181,179
Unpaid principal balance			341,713	327,722
Recorded investment with an allowance			170,142	132,004
Recorded investment without an allowance			147,055	169,341
Hancock Holding Company [Member]
Business Acquisition [Line Items]
Long-term borrowings			234,135	251,573
Whitney Bank [Member]
Business Acquisition [Line Items]
Net of cash acquired		$ 323,000
Cash consideration		326,000
Cash acquired		3,000
Acquisition, Premium amount to pay		41,600
FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			1,132,567	$ 865,000
$450 Debt Due 2020 [Member] | Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			$ 450,000
Maturity year			2020
$200 Debt Due 2025 [Member] | Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			$ 200,000
Maturity year			2025
$260 Debt Due 2026 [Member] | Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			$ 260,000
Maturity year			2026
FNBC Transaction [Member]
Business Acquisition [Line Items]
Acquisition, debt assumed			$ 93,120
Identifiable intangible assets			25,300
Goodwill			124,330
Fair value discount			41,000
Acquisition, debt assumed			4,686
Total Loans			1,368,669
Unpaid principal balance of the loans acquired			1,400,000
Liabilities assumed			2,622,950
Estimated uncollectible amount			31,700
Unpaid principal balance			39,900
Recorded investment with an allowance			15,000
Recorded investment without an allowance			$ 24,900
FNBC I [Member]
Business Acquisition [Line Items]
Acquisition, debt assumed		93,120
Identifiable intangible assets		3,900
Goodwill		103,547
Acquisition, debt assumed		1,607
Total Loans		1,203,092
Useful life			8 years
Liabilities assumed		$ 1,003,647
FNBC I [Member] | Whitney Bank [Member]
Business Acquisition [Line Items]
Number of branches | entity		9
FNBC I [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Liabilities assumed			$ 690,000
FNBC I [Member] | Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			460,000
FNBC I [Member] | Fixed-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Long-term borrowings			93,100
FNBC I [Member] | Fixed-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			$ 51,000
FNBC II [Member]
Business Acquisition [Line Items]
Identifiable intangible assets	$ 21,400
Goodwill	20,783
Acquisition, debt assumed	3,079
Total Loans	165,577
Useful life			11 years
Liabilities assumed	$ 1,619,303
FNBC II [Member] | Whitney Bank [Member]
Business Acquisition [Line Items]
Number of branches | entity	29
Cash received	$ 800,000
Cash acquired	158,000
Number of branches exercised | entity						7
Premium payment	35,000
Unpaid principal balance of the loans acquired, Net liabilities assumed	$ 642,000