Concentrations of Credit Risk - Commercial Real Estate Loans (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Commercial Mortgages
Office	$ 825.2	$ 866.1
Apartment/ Multi-family	623.3	784.8
Retail	631.1	698.1
Industrial/ Warehouse	311.1	359.7
Other	445.6	457.6
Total Commercial Mortgages	2,836.3	3,166.3
Construction, Acquisition and Development Loans	350.8	445.0
Single Family Investment	164.8	179.6
Other Commercial Real Estate Related	130.8	211.6
Total Commercial Real Estate Loans	$ 3,482.7	$ 4,002.5

Commercial Mortgages [Abstract]

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Loans Receivable Commercial Construction Acquisition And Development

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Loans Receivable, Commercial Real Estate, Commercial Mortgage, Apartments And Multi Family

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Loans Receivable, Commercial Real Estate, Commercial Mortgage, Industrial And Warehouse

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Loans Receivable, Commercial Real Estate, Commercial Mortgage, Office

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Loans Receivable, Commercial Real Estate, Commercial Mortgage, Other

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Loans Receivable, Commercial Real Estate, Commercial Mortgage, Retail

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Amount before allowance of mortgage loans on existing land or building. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5066-111524

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(a)(3))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Amount before allowance of commercial loans issued to businesses to acquire, develop, construct, improve, or refinance land or building. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(a))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.1(c))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5066-111524

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