Securities - Schedule of Components of Net Security Losses (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Debt securities:
Gains	$ 0.5	$ 0.1	$ 2.1
Losses	(0.4)	(10.0)	(27.5)
Total debt securities	0.1	(9.9)	(25.4)
Trading debt securities:
Gains	0.1	0.1	0.0
Losses	0.0	0.0	0.0
Total trading debt securities	0.1	0.1	0.0
Net security gains (losses)	$ 0.2	$ (9.8)	(25.4)
Maximum [Member]
Trading debt securities:
Total trading debt securities			$ 0.1

Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27357-111563

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Amount of realized gain (loss) 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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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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Amount of realized gain on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

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-Publisher FASB
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-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27357-111563

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

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-Publisher FASB
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Amount of realized loss on investment in debt security measured at fair value with change in fair value recognized in net income (trading).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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