Mortgage Servicing Rights, Net - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Mortgage Servicing Rights
Primary servicing portfolio	$ 66,300.0	$ 59,900.0
Special servicing portfolio	2,300.0	2,400.0
Discount Rate One
Mortgage Servicing Rights
Decrease in fair value of servicing rights	14.8	11.9
Discount Rate Two
Mortgage Servicing Rights
Decrease in fair value of servicing rights	$ 28.9	$ 23.3
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	$ 527.1	$ 441.7

Mortgage servicing rights.

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- Definition

Percentage of increase in discount rate of servicing rights.

+ References

No definition available.

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- Definition

Primary servicing portfolio.

+ References

No definition available.

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- Definition

Special servicing portfolio.

+ References

No definition available.

+ Details

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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

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122492-111745

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122625-111746

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 45
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

+ Details

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- Definition

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
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122625-111746

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- Definition

Discount rate used to estimate the fair value of servicing assets and servicing liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 2
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=116651436&loc=d3e122596-111746

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