Acquisition (Narrative) (Detail) $ in Thousands		3 Months Ended
	Mar. 10, 2017 USD ($) entity	Mar. 31, 2017 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Acquisition, debt assumed	$ 93,120
Short-term borrowings		$ 2,121,932		$ 1,225,406
Long-term borrowings		525,082		436,280
Identifiable intangible assets	3,900
Goodwill	95,568	716,761		$ 621,193
Revenue		181,691	$ 162,836
Merger-related charges		6,500
Professional Fees		11,276	7,621
Other expense		18,630	$ 16,709
Fair value discount		52,700
Acquisition, debt assumed	$ 1,607
Whitney Bank [Member]
Business Acquisition [Line Items]
Number of branches | entity	9
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]
Acquisition, debt assumed	$ 604,000
Variable-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings		460,000
Fixed-rate Term Notes [Member] | FHLB Borrowings [Member]
Business Acquisition [Line Items]
Short-term borrowings		$ 51,000
Maturity year		2017
Long-term borrowings		$ 93,100
FNBC [Member]
Business Acquisition [Line Items]
Revenue		5,100
Estimated net income		2,900
Professional Fees		4,600
Other expense		1,500
$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
$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
$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
$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