Fair Value Measurements - Assets Measured at Fair Value on Non-Recurring Basis (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 511.3				$ 19.5				$ 511.3	$ 19.5
Impaired loans	61.9				55.2				61.9	55.2
Mortgage servicing rights	10.6				0.0				10.6	0.0
Total	3,971.6				3,296.9				3,971.6	3,296.9
Provision for loan losses (note 5)	7.3	$ 7.8	$ 7.6	$ 5.6	9.9	$ 8.2	$ 6.5	$ 5.4	28.3	30.0	$ 26.0
Repossessed assets	4.2				3.9				4.2	3.9
Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	49.0								49.0
Provision for loan losses (note 5)									29.6	27.8	16.2
Retail Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	12.9								12.9
Provision for loan losses (note 5)									(1.3)	2.2	$ 9.8
Nonoperating Income (Expense) [Member]
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									1.1	0.2
Impaired Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Provision for loan losses (note 5)									9.3	8.6
REO and Repossessed Assets [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Provision for loan losses (note 5)									2.8	1.7
Other Consumer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	333.7								333.7
Other Consumer [Member] | Retail Loans [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0.0				0.0				0.0	0.0
Residential Mortgage [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	19.7				19.5				19.7	19.5
Real estate owned	11.9								11.9
Commercial Real Estate Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Real estate owned	7.3				8.7				7.3	8.7
Commercial Real Estate Loan [Member] | Commercial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	21.0				21.8				21.0	21.8
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	753.4				750.6				753.4	750.6
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	3,218.2				2,546.3				3,218.2	2,546.3
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total	0.0				0.0				0.0	0.0
Fair Value, Measurements, Nonrecurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	61.9				55.2				61.9	55.2
REO and repossessed assets	23.4				18.1				23.4	18.1
Mortgage servicing rights	10.3								10.3
Total	606.9				92.8				606.9	92.8
Fair Value, Measurements, Nonrecurring [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	157.9								157.9
Fair Value, Measurements, Nonrecurring [Member] | Other Consumer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	333.7								333.7
Fair Value, Measurements, Nonrecurring [Member] | Residential Mortgage [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	19.7				19.5				19.7	19.5
Fair Value, Measurements, Nonrecurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0.0				0.0				0.0	0.0
REO and repossessed assets	0.0				0.0				0.0	0.0
Mortgage servicing rights	0.0								0.0
Total	0.0				0.0				0.0	0.0
Fair Value, Measurements, Nonrecurring [Member] | Level 1 [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0.0								0.0
Fair Value, Measurements, Nonrecurring [Member] | Level 1 [Member] | Other Consumer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0.0								0.0
Fair Value, Measurements, Nonrecurring [Member] | Level 1 [Member] | Residential Mortgage [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0.0				0.0				0.0	0.0
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	0.0				0.0				0.0	0.0
REO and repossessed assets	0.0				0.0				0.0	0.0
Mortgage servicing rights	0.0								0.0
Total	19.7				19.5				19.7	19.5
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0.0								0.0
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member] | Other Consumer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	0.0								0.0
Fair Value, Measurements, Nonrecurring [Member] | Level 2 [Member] | Residential Mortgage [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	19.7				19.5				19.7	19.5
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans	61.9				55.2				61.9	55.2
REO and repossessed assets	23.4				18.1				23.4	18.1
Mortgage servicing rights	10.3								10.3
Total	587.2				73.3				587.2	73.3
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Commercial Loan [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	157.9								157.9
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Other Consumer [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	333.7								333.7
Fair Value, Measurements, Nonrecurring [Member] | Level 3 [Member] | Residential Mortgage [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Loans held-for-sale	$ 0.0				$ 0.0				$ 0.0	$ 0.0