Investment Securities - Carrying Amounts and Fair Values of Investment Securities (Detail) - USD ($) $ in Millions	Jun. 30, 2021		Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	$ 193.8	[1]	$ 167.3	[2]
Debt Securities, Held-to-maturity	968.7		568.8
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	46.1		43.0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(2.1)		0.0
Debt Securities, Held-to-maturity, Fair Value	1,012.7		611.8
Securities available for sale Total, Amortized Cost	6,529.2		4,586.4
Securities available-for-sale, Gross Unrealized Gains	139.8		137.6
Securities available-for-sale, Gross Unrealized (Losses)	(53.8)		(15.5)
Securities available for sale Total, Estimated Fair Value	6,615.2	[1]	4,708.5	[2]
Equity Securities, FV-NI, Cost	185.0		160.1
Equity Securities, FV-NI, Unrealized Gain	9.0		7.6
Equity Securities, FV-NI, Unrealized Loss	0.2		0.4
Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Amortized Cost	81.2		80.8
Securities available-for-sale, Gross Unrealized Gains	1.7		3.8
Securities available-for-sale, Gross Unrealized (Losses)	(1.4)		0.0
Securities available for sale Total, Estimated Fair Value	81.5		84.6
Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Amortized Cost	361.9		271.1
Securities available-for-sale, Gross Unrealized Gains	10.3		4.8
Securities available-for-sale, Gross Unrealized (Losses)	(8.8)		(5.7)
Securities available for sale Total, Estimated Fair Value	363.4		270.2
Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	230.2
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	0.0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	0.0
Debt Securities, Held-to-maturity, Fair Value	230.2
Securities available for sale Total, Amortized Cost	1,712.3		1,461.7
Securities available-for-sale, Gross Unrealized Gains	8.3		15.7
Securities available-for-sale, Gross Unrealized (Losses)	(10.2)		(0.5)
Securities available for sale Total, Estimated Fair Value	1,710.4		1,476.9
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Amortized Cost	2,112.9		1,462.5
Securities available-for-sale, Gross Unrealized Gains	20.9		27.9
Securities available-for-sale, Gross Unrealized (Losses)	(28.1)		(3.8)
Securities available for sale Total, Estimated Fair Value	2,105.7		1,486.6
Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	738.5		568.8
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	46.1
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	(2.1)
Debt Securities, Held-to-maturity, Fair Value	782.5
Securities available for sale Total, Amortized Cost	1,260.7		1,109.3
Securities available-for-sale, Gross Unrealized Gains	87.2		78.1
Securities available-for-sale, Gross Unrealized (Losses)	(0.5)		0.0
Securities available for sale Total, Estimated Fair Value	1,347.4		1,187.4
US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Amortized Cost	9.3
Securities available-for-sale, Gross Unrealized Gains	0.0
Securities available-for-sale, Gross Unrealized (Losses)	0.0
Securities available for sale Total, Estimated Fair Value	9.3
CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	57.0		53.4
Equity Securities, FV-NI, Cost	57.2		53.1
Equity Securities, FV-NI, Unrealized Gain	0.0		0.3
Equity Securities, FV-NI, Unrealized Loss	0.2		0.0
Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	136.8		113.9
Equity Securities, FV-NI, Cost	127.8		107.0
Equity Securities, FV-NI, Unrealized Gain	9.0		7.3
Equity Securities, FV-NI, Unrealized Loss	0.0		0.4
Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Amortized Cost	936.8		146.9
Securities available-for-sale, Gross Unrealized Gains	0.6		0.0
Securities available-for-sale, Gross Unrealized (Losses)	(0.3)		0.0
Securities available for sale Total, Estimated Fair Value	937.1		146.9
Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Amortized Cost	54.1		54.1
Securities available-for-sale, Gross Unrealized Gains	10.8		7.3
Securities available-for-sale, Gross Unrealized (Losses)	(4.5)		(5.5)
Securities available for sale Total, Estimated Fair Value	60.4		55.9
AAA [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0	[1]	0.0	[2]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	1,714.3	[1]	1,429.8	[2]
AAA [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
AAA [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
AAA [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	1,621.5		1,385.5
AAA [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
AAA [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0.0		0.0
Securities available for sale Total, Estimated Fair Value	43.5		44.3
AAA [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	9.3
AAA [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
AAA [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
AAA [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	40.0		0.0
AAA [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
Split-rated AAA/AA Plus [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	27.5	[1]	27.8	[2]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	2,246.4	[1]	1,628.5	[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]
Securities available for sale Total, Estimated Fair Value	81.5		84.6
Split-rated AAA/AA Plus [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		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
Securities available for sale Total, Estimated Fair Value	0.0		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]
Securities available for sale Total, Estimated Fair Value	2,105.7		1,486.6
Split-rated AAA/AA Plus [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0.0		0.0
Securities available for sale Total, Estimated Fair Value	59.2		57.3
Split-rated AAA/AA Plus [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0
Split-rated AAA/AA Plus [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	27.5		27.8
Split-rated AAA/AA Plus [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
Split-rated AAA/AA Plus [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
Split-rated AAA/AA Plus [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
AA Plus to AA- [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0	[1]	0.0	[2]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	1,217.8	[1]	715.9	[2]
AA Plus to AA- [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
AA Plus to AA- [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	18.6		19.2
AA Plus to AA- [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	87.7		90.1
AA Plus to AA- [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		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		0.0
Securities available for sale Total, Estimated Fair Value	522.0		454.7
AA Plus to AA- [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0
AA Plus to AA- [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
AA Plus to AA- [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
AA Plus to AA- [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	577.4		139.6
AA Plus to AA- [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	12.1		12.3
A Plus to A- [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0	[1]	0.0	[2]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	1,039.5	[1]	634.8	[2]
A Plus to A- [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
A Plus to A- [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	26.8		28.1
A Plus to A- [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	0.1		0.1
A Plus to A- [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		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		0.0
Securities available for sale Total, Estimated Fair Value	692.9		599.3
A Plus to A- [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0
A Plus to A- [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
A Plus to A- [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
A Plus to A- [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	319.7		7.3
A Plus to A- [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
BBB Plus to BBB- [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	81.7	[1]	73.2	[2]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	329.6	[1]	223.9	[2]
BBB Plus to BBB- [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
BBB Plus to BBB- [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	299.0		194.5
BBB Plus to BBB- [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	0.3		0.3
BBB Plus to BBB- [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		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		0.0
Securities available for sale Total, Estimated Fair Value	0.0		0.0
BBB Plus to BBB- [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0
BBB Plus to BBB- [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
BBB Plus to BBB- [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	81.7		73.2
BBB Plus to BBB- [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
BBB Plus to BBB- [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	30.3		29.1
BB Plus and below [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	39.6	[1]	39.0	[2]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	29.2	[1]	36.2	[2]
BB Plus and below [Member] | Commercial Mortgage Backed Securities Issued By US Government Sponsored Enterprise [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
BB Plus and below [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	19.0		28.4
BB Plus and below [Member] | Residential Mortgage Backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	0.0
Securities available for sale Total, Estimated Fair Value	0.8		0.9
BB Plus and below [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		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		0.0
Securities available for sale Total, Estimated Fair Value	0.0		0.0
BB Plus and below [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0
BB Plus and below [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	0.0		0.0
BB Plus and below [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	39.6		39.0
BB Plus and below [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
BB Plus and below [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	9.4		6.9
Corporate Credit Quality Indicator Unrated [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	45.0	[1]	27.3	[2]
Debt Securities, Held-to-maturity	968.7
Securities available for sale Total, Estimated Fair Value	38.4	[1]	39.4	[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]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
Corporate Credit Quality Indicator Unrated [Member] | Corporate Bond Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		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	230.2
Securities available for sale Total, Estimated Fair Value	0.0		0.0
Corporate Credit Quality Indicator Unrated [Member] | Mortgage-backed Securities, Issued by US Government Sponsored Enterprises [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
Corporate Credit Quality Indicator Unrated [Member] | Nontaxable Municipal Bonds [Member]
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Held-to-maturity	738.5		568.8
Securities available for sale Total, Estimated Fair Value	29.8		31.8
Corporate Credit Quality Indicator Unrated [Member] | US Treasury Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0
Corporate Credit Quality Indicator Unrated [Member] | CRA investments [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	29.5		25.6
Corporate Credit Quality Indicator Unrated [Member] | Preferred Stock [Member]
Debt Securities, Available-for-sale [Line Items]
Investment securities - equity	15.5		1.7
Corporate Credit Quality Indicator Unrated [Member] | Collateralized Loan Obligations
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	0.0		0.0
Corporate Credit Quality Indicator Unrated [Member] | Other Security Investments
Debt Securities, Available-for-sale [Line Items]
Securities available for sale Total, Estimated Fair Value	$ 8.6		$ 7.6

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

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