Summary of Significant Accounting Policies - Additional Information (Detail)			3 Months Ended								12 Months Ended
	Jan. 01, 2020 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2020	Dec. 31, 2019 USD ($) Rating Reporting_Unit	Dec. 31, 2018 USD ($) Reporting_Unit	Dec. 31, 2017 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Number of days of non accrual status												90 days
Accruing interest total			$ 0				$ 0					$ 0	$ 0	$ 0
Collateral values/LTV ratios, minimum												70.00%
Borrower credit scores, minimum | Rating												680
Non-interest income			$ 124,200,000	$ 106,000,000.0	$ 106,300,000	$ 94,600,000	$ 88,700,000	$ 92,300,000	$ 94,900,000	$ 90,400,000		$ 431,100,000	$ 366,400,000	352,900,000
Non-interest income percentage			67.00%				74.00%					67.00%	74.00%
Number of reporting units | Reporting_Unit												3	3
Operating lease			$ 307,900,000									$ 307,900,000
Lease ROU assets			276,600,000									276,600,000
Retained earnings			$ 1,512,800,000				$ 1,284,800,000					1,512,800,000	$ 1,284,800,000
Operating Segments [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-interest income												402,400,000	364,500,000	348,400,000
Significant Revenue Streams [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-interest income												$ 288,000,000	270,000,000
Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Operating lease		$ 268,800,000
Lease ROU assets		$ 248,500,000
Increase (decrease) in risk-based capital ratios		(0.10%)
Accounting Standards Update 2018-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in risk-based capital ratios										0.11%
Accounting Standards Update 2018-02 [Member] | Retained Earnings [Member]
Summary Of Significant Accounting Policies [Line Items]
Cumulative effect on retained earnings										$ 37,900,000
Accounting Standards Update 2016-13 [Member] | Subsequent Event [Member] | Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
Retained earnings	$ 60,000,000
Accounting Standards Update 2016-13 [Member] | Subsequent Event [Member] | PCI [Member] | Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
Adjustment to amortized cost basis	$ (30,000,000)
Trade Name [Member] | Accelerated Amortization [Member]
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, useful life, (in years)												20 years
Trade Name [Member] | Straight Line Amortization [Member]
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, useful life, (in years)												5 years
Commercial Banking Loan [Member] | Operating Segments [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-interest income												$ 206,500,000	$ 177,800,000	165,000,000.0
Non-interest income percentage			40.00%				40.00%					40.00%	40.00%
Retail Banking [Member] | Operating Segments [Member]
Summary Of Significant Accounting Policies [Line Items]
Non-interest income												$ 195,900,000	$ 186,700,000	$ 183,400,000
Non-interest income percentage			30.00%				30.00%					30.00%	30.00%
Leasehold Improvements [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets, (in years)												10 years
Building [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets, (in years)												40 years
Furniture and Fixtures [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets, (in years)												10 years
Minimum [Member]
Summary Of Significant Accounting Policies [Line Items]
Loan agreement												6 months
Non accrual commercial loans			$ 1,000,000									$ 1,000,000
Minimum [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in risk-based capital ratios		(0.05%)
Minimum [Member] | Accounting Standards Update 2016-13 [Member] | Subsequent Event [Member] | Scenario, Forecast [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in regulatory capital ratio											(0.10%)
Minimum [Member] | Core Deposit Intangibles [Member]
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, useful life, (in years)												6 years
Minimum [Member] | Customer Relationships [Member]
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, useful life, (in years)												10 years
Minimum [Member] | Data Processing and Other Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets, (in years)												3 years
Minimum [Member] | Computer Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets, (in years)												3 years
Maximum [Member]
Summary Of Significant Accounting Policies [Line Items]
Term of contract with customers												1 year
Maximum [Member] | Accounting Standards Update 2016-02 [Member]
Summary Of Significant Accounting Policies [Line Items]
Increase (decrease) in risk-based capital ratios		(0.10%)
Maximum [Member] | Core Deposit Intangibles [Member]
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, useful life, (in years)												10 years
Maximum [Member] | Customer Relationships [Member]
Summary Of Significant Accounting Policies [Line Items]
Finite-lived intangible assets, useful life, (in years)												15 years
Maximum [Member] | Data Processing and Other Equipment [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets, (in years)												5 years
Maximum [Member] | Computer Software [Member]
Summary Of Significant Accounting Policies [Line Items]
Estimated useful lives of assets, (in years)												5 years