Investment Securities - Carrying Amounts and Fair Values of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2022		Dec. 31, 2021
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	$ 164,700	[1]	$ 158,500	[2]
Debt Securities, Held-to-maturity	1,161,000		1,107,100
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	18,200		43,200
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(39,300)		(3,900)
Debt Securities, Held-to-maturity, Fair Value	1,139,900		1,146,400
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	334,600		81,300
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	21,300		103,600
Debt Securities, Available-for-sale	6,852,200	[1]	6,188,800	[2]
Securities available for sale Total, Amortized Cost	7,165,500		6,166,500
Equity Securities, FV-NI, Cost	165,100		152,200
Equity Securities, FV-NI, Unrealized Gain	4,900		7,700
Equity Securities, FV-NI, Unrealized Loss	5,300		1,400
Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	2,800		600
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	100		900
Debt Securities, Available-for-sale	62,200		68,500
Securities available for sale Total, Amortized Cost	64,900		68,200
Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	11,000		9,000
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,700		9,200
Debt Securities, Available-for-sale	415,300		382,900
Securities available for sale Total, Amortized Cost	424,600		382,700
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	211,100		216,900
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0		0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(18,900)		(1,600)
Debt Securities, Held-to-maturity, Fair Value	192,200		215,300
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	101,100		24,200
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	300		3,500
Debt Securities, Available-for-sale	1,343,200		1,508,000
Securities available for sale Total, Amortized Cost	1,444,000		1,528,700
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	169,000		41,400
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	200		7,300
Debt Securities, Available-for-sale	1,905,000		1,993,400
Securities available for sale Total, Amortized Cost	2,073,800		2,027,500
Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	949,900		890,200
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	18,200		43,200
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(20,400)		(2,300)
Debt Securities, Held-to-maturity, Fair Value	947,700		931,100
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	37,400		1,200
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8,600		71,200
Debt Securities, Available-for-sale	992,900		1,215,100
Securities available for sale Total, Amortized Cost	1,021,700		1,145,100
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax			0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax			0
Debt Securities, Available-for-sale			13,000
Securities available for sale Total, Amortized Cost			13,000
CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	52,300		44,600
Equity Securities, FV-NI, Cost	54,100		45,100
Equity Securities, FV-NI, Unrealized Gain	0		0
Equity Securities, FV-NI, Unrealized Loss	1,800		500
Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	112,400		113,900
Equity Securities, FV-NI, Cost	111,000		107,100
Equity Securities, FV-NI, Unrealized Gain	4,900		7,700
Equity Securities, FV-NI, Unrealized Loss	3,500		900
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	7,200		300
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,200		500
Debt Securities, Available-for-sale	2,055,300		926,200
Securities available for sale Total, Amortized Cost	2,061,300		926,000
Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	6,100		4,600
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	9,200		11,000
Debt Securities, Available-for-sale	78,300		81,700
Securities available for sale Total, Amortized Cost	75,200		75,300
AAA [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	[1]	0	[2]
Debt Securities, Held-to-maturity	0
Debt Securities, Available-for-sale	1,402,500	[1]	1,507,800	[2]
AAA [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
AAA [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
AAA [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	1,261,300		1,419,600
AAA [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
AAA [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	17,700		43,200
AAA [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale			0
AAA [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
AAA [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
AAA [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	123,500		45,000
AAA [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
Split-rated AAA/AA Plus [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	26,200	[1]	27,300	[2]
Debt Securities, Held-to-maturity	0
Debt Securities, Available-for-sale	1,983,800	[1]	2,115,300	[2]
Split-rated AAA/AA Plus [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	62,200		68,500
Split-rated AAA/AA Plus [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
Split-rated AAA/AA Plus [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	0		0
Split-rated AAA/AA Plus [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	1,905,000		1,993,400
Split-rated AAA/AA Plus [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	16,600		40,400
Split-rated AAA/AA Plus [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale			13,000
Split-rated AAA/AA Plus [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	26,200		27,300
Split-rated AAA/AA Plus [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
Split-rated AAA/AA Plus [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
Split-rated AAA/AA Plus [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
AA Plus to AA- [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	[1]	0	[2]
Debt Securities, Held-to-maturity	0
Debt Securities, Available-for-sale	2,211,200	[1]	1,204,200	[2]
AA Plus to AA- [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
AA Plus to AA- [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
AA Plus to AA- [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	81,000		87,400
AA Plus to AA- [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
AA Plus to AA- [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	443,300		469,100
AA Plus to AA- [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale			0
AA Plus to AA- [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
AA Plus to AA- [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
AA Plus to AA- [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	1,676,100		635,600
AA Plus to AA- [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	10,800		12,100
A Plus to A- [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0	[1]	0	[2]
Debt Securities, Held-to-maturity	0
Debt Securities, Available-for-sale	795,800	[1]	929,800	[2]
A Plus to A- [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
A Plus to A- [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	48,000		45,300
A Plus to A- [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	0		0
A Plus to A- [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
A Plus to A- [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	482,300		628,900
A Plus to A- [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale			0
A Plus to A- [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
A Plus to A- [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
A Plus to A- [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	255,700		245,600
A Plus to A- [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	9,800		10,000
BBB Plus to BBB- [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	78,000	[1]	79,300	[2]
Debt Securities, Held-to-maturity	0
Debt Securities, Available-for-sale	398,200	[1]	349,800	[2]
BBB Plus to BBB- [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
BBB Plus to BBB- [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	367,300		318,700
BBB Plus to BBB- [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	800		900
BBB Plus to BBB- [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
BBB Plus to BBB- [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	0		0
BBB Plus to BBB- [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale			0
BBB Plus to BBB- [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
BBB Plus to BBB- [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	78,000		79,300
BBB Plus to BBB- [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
BBB Plus to BBB- [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	30,100		30,200
BB Plus and below [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	19,200	[1]	20,200	[2]
Debt Securities, Held-to-maturity	0
Debt Securities, Available-for-sale	8,500	[1]	28,500	[2]
BB Plus and below [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
BB Plus and below [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		18,900
BB Plus and below [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	0		0
BB Plus and below [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
BB Plus and below [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0		0
Debt Securities, Available-for-sale	0		0
BB Plus and below [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale			0
BB Plus and below [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0		0
BB Plus and below [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	19,200		20,200
BB Plus and below [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
BB Plus and below [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	8,500		9,600
Corporate Credit Quality Indicator Unrated [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	41,300	[1]	31,700	[2]
Debt Securities, Held-to-maturity	1,161,000		1,107,100
Debt Securities, Available-for-sale	52,200	[1]	53,400	[2]
Corporate Credit Quality Indicator Unrated [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
Corporate Credit Quality Indicator Unrated [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
Corporate Credit Quality Indicator Unrated [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	211,100		216,900
Debt Securities, Available-for-sale	100		100
Corporate Credit Quality Indicator Unrated [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
Corporate Credit Quality Indicator Unrated [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	949,900		890,200
Debt Securities, Available-for-sale	33,000		33,500
Corporate Credit Quality Indicator Unrated [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale			0
Corporate Credit Quality Indicator Unrated [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	26,100		17,300
Corporate Credit Quality Indicator Unrated [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	15,200		14,400
Corporate Credit Quality Indicator Unrated [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	0		0
Corporate Credit Quality Indicator Unrated [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale	$ 19,100		$ 19,800

[1] ratings differ, the Company uses an average of the available ratings by major credit agencies.

[2] For rated securities, if ratings differ, the Company uses an average of the available ratings by major credit agencies.