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Mortgage Servicing Rights, Net - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2021	Dec. 31, 2020
Mortgage Servicing Rights
Primary servicing portfolio	$ 67,900.0	$ 66,300.0
Special servicing portfolio	2,200.0	2,300.0
Discount Rate One
Mortgage Servicing Rights
Decrease in fair value of servicing rights	16.3	14.8
Discount Rate Two
Mortgage Servicing Rights
Decrease in fair value of servicing rights	$ 31.8	$ 28.9
Minimum
Mortgage Servicing Rights
Discount rates used in measuring fair value (in percentage)	6.10%	6.10%
Minimum | Discount Rate One
Mortgage Servicing Rights
Increase in discount rate (in percentage)	1.00%
Maximum
Mortgage Servicing Rights
Discount rates used in measuring fair value (in percentage)	13.50%	13.50%
Maximum | Discount Rate Two
Mortgage Servicing Rights
Increase in discount rate (in percentage)	2.00%
Mortgage Servicing Rights
Mortgage Servicing Rights
Estimated fair value of MSRs	$ 570.4	$ 527.1

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- Definition

Mortgage servicing rights.

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Percentage of increase in discount rate of servicing rights.

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Primary servicing portfolio.

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Special servicing portfolio.

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- Definition

Fair value of an asset representing net future revenue from contractually specified servicing fees, late charges, and other ancillary revenues, in excess of future costs related to servicing arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
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-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122492-111745

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
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The increase (decrease) in the fair value of servicing assets that are subsequently measured at fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
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Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

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-Publisher FASB
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