Cash and Cash Equivalents and Securities - Available-for-Sale and Held-to-Maturity Debt Securities Gains (Losses) (Detail) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Debt securities available-for-sale:
Amortized Cost	$ 4,156.4	$ 3,545.8
Allowance for Credit Losses	0.0
Gross Unrealized Gains	120.3	25.4
Gross Unrealized Losses	(0.1)	(6.9)
Fair Value	4,276.6	3,564.3
Debt securities held-to-maturity:
Amortized Cost	3,863.4	3,869.2
Allowance for Credit Losses	1.9
Gross Unrealized Gains	202.7	151.5
Gross Unrealized Losses	(2.8)	(0.7)
Fair Value	4,065.2	4,020.0
U.S. Treasury and agency
Debt securities available-for-sale:
Amortized Cost	731.3	689.5
Allowance for Credit Losses	0.0
Gross Unrealized Gains	15.9	0.4
Gross Unrealized Losses	0.0	(2.8)
Fair Value	747.2	687.1
Debt securities held-to-maturity:
Amortized Cost	0.0
Fair Value	0.0
GSE mortgage-backed securities
Debt securities available-for-sale:
Amortized Cost	3,425.1	2,856.3
Allowance for Credit Losses	0.0
Gross Unrealized Gains	104.4	25.0
Gross Unrealized Losses	(0.1)	(4.1)
Fair Value	3,529.4	2,877.2
Debt securities held-to-maturity:
Amortized Cost	1,190.8	1,271.4
Allowance for Credit Losses	0.0
Gross Unrealized Gains	49.7	8.0
Gross Unrealized Losses	0.0	(0.3)
Fair Value	1,240.5	1,279.1
State and municipal
Debt securities available-for-sale:
Amortized Cost	0.0
Fair Value	0.0
Debt securities held-to-maturity:
Amortized Cost	2,574.9	2,503.9
Allowance for Credit Losses	0.1
Gross Unrealized Gains	151.4	142.0
Gross Unrealized Losses	(1.5)	(0.4)
Fair Value	2,724.9	2,645.5
Corporate
Debt securities available-for-sale:
Amortized Cost	0.0
Fair Value	0.0
Debt securities held-to-maturity:
Amortized Cost	96.2	92.4
Allowance for Credit Losses	1.8
Gross Unrealized Gains	1.6	1.5
Gross Unrealized Losses	(1.3)	0.0
Fair Value	98.3	93.9
Other
Debt securities available-for-sale:
Amortized Cost	0.0
Fair Value	0.0
Debt securities held-to-maturity:
Amortized Cost	1.5	1.5
Allowance for Credit Losses	0.0
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Fair Value	$ 1.5	$ 1.5

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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