Long-Term Debt - Summary of Long-term Debt (Detail) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014
Debt Instrument [Line Items]
Long-term debt	$ 32,078	$ 32,260
Parent Company [Member]
Debt Instrument [Line Items]
Long-term debt	11,453	13,189
Parent Company [Member] \| Capitalized Lease Obligations, Mortgage Indebtedness and Other [Member]
Debt Instrument [Line Items]
Long-term debt	204	190
Subsidiaries [Member]
Debt Instrument [Line Items]
Long-term debt	20,625	19,071
Subsidiaries [Member] \| Capitalized Lease Obligations, Mortgage Indebtedness and Other [Member]
Debt Instrument [Line Items]
Long-term debt	$ 755	607
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	2.95%
Debt Instrument Maturity Year, start	Jan. 01, 2022
Debt Instrument Maturity Year, End	Dec. 31, 2022
Long-term debt	$ 1,300	1,300
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Debt One [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.60%
Debt Instrument Maturity Year, start	Jan. 01, 2024
Debt Instrument Maturity Year, End	Dec. 31, 2024
Long-term debt	$ 1,000	1,000
Fixed Rate [Member] \| Parent Company [Member] \| Subordinated Notes Two [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.50%
Debt Instrument Maturity Year, start	Jan. 01, 2026
Debt Instrument Maturity Year, End	Dec. 31, 2026
Long-term debt	$ 199	199
Fixed Rate [Member] \| Parent Company [Member] \| Medium-term Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	1.65%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	4.125%
Debt Instrument Maturity Year, start	Jan. 01, 2016
Debt Instrument Maturity Year, End	Dec. 31, 2024
Long-term debt	$ 7,500	9,250
Fixed Rate [Member] \| Parent Company [Member] \| Junior Subordinated Debentures [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.442%
Debt Instrument Maturity Year, start	Jan. 01, 2016
Debt Instrument Maturity Year, End	Dec. 31, 2016
Long-term debt	$ 500	500
Fixed Rate [Member] \| Subsidiaries [Member] \| Subordinated Notes Three [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	4.80%
Debt Instrument Maturity Year, start	Jan. 01, 2015
Debt Instrument Maturity Year, End	Dec. 31, 2015
Long-term debt		500
Fixed Rate [Member] \| Subsidiaries [Member] \| Subordinated Notes Four [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	3.778%
Debt Instrument Maturity Year, start	Jan. 01, 2020
Debt Instrument Maturity Year, End	Dec. 31, 2020
Long-term debt		500
Fixed Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	1.25%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	8.25%
Debt Instrument Maturity Year, start	Jan. 01, 2017
Debt Instrument Maturity Year, End	Dec. 31, 2026
Long-term debt	$ 11	11
Fixed Rate [Member] \| Subsidiaries [Member] \| Bank Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	1.10%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	2.80%
Debt Instrument Maturity Year, start	Jan. 01, 2017
Debt Instrument Maturity Year, End	Dec. 31, 2025
Long-term debt	$ 5,850	4,050
Floating Rate [Member] \| Parent Company [Member] \| Medium-term Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.72%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	0.852%
Debt Instrument Maturity Year, start	Jan. 01, 2018
Debt Instrument Maturity Year, End	Dec. 31, 2019
Long-term debt	$ 750	750
Floating Rate [Member] \| Subsidiaries [Member] \| Federal Home Loan Bank Advances [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.335%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	0.847%
Debt Instrument Maturity Year, start	Jan. 01, 2016
Debt Instrument Maturity Year, End	Dec. 31, 2025
Long-term debt	$ 9,081	7,334
Floating Rate [Member] \| Subsidiaries [Member] \| Bank Notes [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Minimum	0.016%
Debt Instrument, Interest Rate, Stated Percentage Rate Range, Maximum	0.803%
Debt Instrument Maturity Year, start	Jan. 01, 2016
Debt Instrument Maturity Year, End	Dec. 31, 2055
Long-term debt	$ 4,928	$ 6,069

Contractual interest rate for funds borrowed, under the debt agreement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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-Name Accounting Standards Codification
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Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
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Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20, 22
-Article 5

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