Fair Value Measurements - Assets on Non-Recurring Basis (Detail) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 19,200,000		$ 511,300,000
Impaired loans			61,900,000
Fair value of total assets measured at fair value on a recurring basis	5,271,200,000		3,971,600,000
Repossessed assets	4,600,000		4,200,000
Provision for credit losses	33,500,000	$ 5,600,000
Impaired Loans
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Provision for credit losses		600,000
REO and Repossessed Assets
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Provision for credit losses	1,200,000	300,000
Nonoperating Income (Expense)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Write downs and net loss on sale of foreclosed/repossessed assets charged to non-interest expense total	400,000	100,000
Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate owned	7,300,000		7,300,000
Residential mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value adjustments	0	$ 0
Real estate owned	9,500,000		11,900,000
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of total assets measured at fair value on a recurring basis	796,100,000		753,400,000
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of total assets measured at fair value on a recurring basis	4,475,100,000		3,218,200,000
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of total assets measured at fair value on a recurring basis	0		0
Nonrecurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	19,200,000
Impaired loans			61,900,000
REO and repossessed assets	21,400,000		23,400,000
Servicing Asset	7,700,000		10,300,000
Fair value of total assets measured at fair value on a recurring basis	48,300,000		606,900,000
Fair value adjustments	3,000,000.0
Nonrecurring | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			157,900,000
Nonrecurring | Other consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			333,700,000
Nonrecurring | Residential mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	19,200,000		19,700,000
Nonrecurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0
Impaired loans			0
REO and repossessed assets	0		0
Servicing Asset	0		0
Fair value of total assets measured at fair value on a recurring basis	0		0
Nonrecurring | Level 1 | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			0
Nonrecurring | Level 1 | Other consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			0
Nonrecurring | Level 1 | Residential mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			0
Nonrecurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	19,200,000
Impaired loans			0
REO and repossessed assets	0		0
Servicing Asset	0		0
Fair value of total assets measured at fair value on a recurring basis	19,200,000		19,700,000
Nonrecurring | Level 2 | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			0
Nonrecurring | Level 2 | Other consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			0
Nonrecurring | Level 2 | Residential mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			19,700,000
Nonrecurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0
Impaired loans			61,900,000
REO and repossessed assets	21,400,000		23,400,000
Servicing Asset	7,700,000		10,300,000
Fair value of total assets measured at fair value on a recurring basis	$ 29,100,000		587,200,000
Nonrecurring | Level 3 | Commercial real estate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			157,900,000
Nonrecurring | Level 3 | Other consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			333,700,000
Nonrecurring | Level 3 | Residential mortgage
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale			$ 0