Acquisitions (Narrative) (Details)			3 Months Ended		9 Months Ended
	Apr. 28, 2017 USD ($) entity	Mar. 10, 2017 USD ($) entity	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Business Acquisition [Line Items]
Short-term borrowings			$ 1,737,151,000		$ 1,737,151,000		$ 1,225,406,000
Long-term borrowings			331,179,000		331,179,000		436,280,000
Goodwill			739,403,000		739,403,000		621,193,000
Revenue			202,857,000	$ 163,513,000	584,265,000	$ 491,318,000
Professional Fees			10,062,000	6,584,000	31,691,000	21,917,000
Other expense			21,760,000	22,379,000	68,057,000	55,561,000
Loan expected to be uncollectible			223,122,000	$ 236,061,000	223,122,000	$ 236,061,000	229,418,000	$ 181,179,000
Unpaid principal balance			312,397,000		312,397,000		327,722,000
Recorded investment with an allowance			194,279,000		194,279,000		132,004,000
Recorded investment without an allowance			108,118,000		108,118,000		$ 169,341,000
Whitney Bank [Member]
Business Acquisition [Line Items]
Number of branches | entity		9
Net of cash acquired		$ 323,000,000
Cash consideration		326,000,000
Cash acquired		3,000,000
Acquisition, Premium amount to pay		41,600,000
FNBC Transaction [Member]
Business Acquisition [Line Items]
Acquisition, debt assumed			93,120,000		93,120,000
Identifiable intangible assets			25,300,000		25,300,000
Goodwill			118,210,000		118,210,000
Fair value discount					41,000,000
Acquisition, debt assumed			4,686,000		4,686,000
Total Loans			1,377,100,000		1,377,100,000
Unpaid principal balance of the loans acquired			1,400,000,000		1,400,000,000
Liabilities assumed			2,622,950,000		2,622,950,000
Estimated uncollectible amount			31,700,000		31,700,000
Unpaid principal balance			39,900,000		39,900,000
Recorded investment with an allowance			23,400,000		23,400,000
Recorded investment without an allowance			16,500,000		$ 16,500,000
FNBC I [Member]
Business Acquisition [Line Items]
Acquisition, debt assumed		93,120,000
Identifiable intangible assets		3,900,000
Goodwill		96,063,000
Acquisition, debt assumed		1,607,000
Total Loans		1,211,523,000
Useful life					8 years
Liabilities assumed		1,003,647,000
FNBC I [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Liabilities assumed		$ 604,000,000
FNBC I [Member] | Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			460,000,000		$ 460,000,000
FNBC I [Member] | Fixed-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			51,000,000		51,000,000
Long-term borrowings			93,100,000		93,100,000
FNBC I [Member] | $200 Debt Due 2025 [Member] | Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			200,000,000		$ 200,000,000
Maturity year					2025
FNBC I [Member] | $260 Debt Due 2026 [Member] | Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings			260,000,000		$ 260,000,000
Maturity year					2026
FNBC I [Member] | $88 Debt Due 2018 [Member] | Fixed-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Maturity year					2018
Long-term borrowings			88,000,000		$ 88,000,000
FNBC I [Member] | $3.2 Debt Due 2019 [Member] | Fixed-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Maturity year					2019
Long-term borrowings			3,200,000		$ 3,200,000
FNBC I [Member] | $1.9 Debt Due 2023 [Member] | Fixed-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Maturity year					2023
Long-term borrowings			1,900,000		$ 1,900,000
FNBC II [Member]
Business Acquisition [Line Items]
Long-term borrowings			$ 0		$ 0
Identifiable intangible assets	$ 21,400,000
Goodwill	22,147,000
Acquisition, debt assumed	3,079,000
Total Loans	165,577,000
Useful life					11 years
Liabilities assumed	$ 1,619,303,000
FNBC II [Member] | Whitney Bank [Member]
Business Acquisition [Line Items]
Number of branches | entity	29
Cash received	$ 800,000,000
Cash acquired	158,000,000
Premium payment	35,000,000
Unpaid principal balance of the loans acquired, Net liabilities assumed	$ 642,000,000
FNBC II [Member] | Louisiana [Member] | Whitney Bank [Member]
Business Acquisition [Line Items]
Number of branches | entity	24
FNBC II [Member] | Florida [Member] | Whitney Bank [Member]
Business Acquisition [Line Items]
Number of branches | entity	5