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Mortgage Servicing Rights, Net - Additional Information (Detail) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)
Mortgage Servicing Rights [Line Items]
Primary servicing portfolio	$ 56,100.0	$ 54,200.0
Special servicing portfolio	$ 3,200.0	$ 3,800.0
Minimum [Member] | Measurement Input, Discount Rate [Member]
Mortgage Servicing Rights [Line Items]
Discount rate	3.0	3.0
Maximum [Member] | Measurement Input, Discount Rate [Member]
Mortgage Servicing Rights [Line Items]
Discount rate	13.5	13.5
Discount Rate One [Member]
Mortgage Servicing Rights [Line Items]
Increase in discount rate	1.00%	1.00%
Decrease in fair value of servicing rights	$ (13.5)	$ (11.8)
Discount Rate Two [Member]
Mortgage Servicing Rights [Line Items]
Increase in discount rate	2.00%	2.00%
Decrease in fair value of servicing rights	$ (25.5)	$ (23.0)
Mortgage Servicing Rights [Member]
Mortgage Servicing Rights [Line Items]
Estimated fair value of MSRs	$ 444.9	$ 418.1

X

- Definition

Mortgage servicing rights.

+ References

No definition available.

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- Definition

Percentage of increase in discount rate of servicing rights.

+ References

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- Definition

Primary servicing portfolio.

+ References

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X

- Definition

Special servicing portfolio.

+ References

No definition available.

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X

- Definition

Value of input used to measure alternative investment.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258

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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
-URI http://asc.fasb.org/extlink&oid=6469459&loc=d3e122501-111745

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=68078379&loc=d3e122625-111746

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 50
-Section 50
-Paragraph 3
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