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Schedule of Gross Unrealized Losses and Fair Values of Securities (Detail) - USD ($) $ in Millions | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
Investment [Line Items] |  |
Less than 12 months, Fair Value | $ 1,648 | $ 4,326
Less than 12 months, Gross Unrealized Losses | (24) | (84)
12 months or more, Fair Value | 521 | 484
12 months or more, Gross Unrealized Losses | (40) | (42)
Total, Fair Value | 2,169 | 4,810
Total, Gross Unrealized Losses | (64) | (126)
Debt Securities | Tax-exempt municipal securities |  |
Investment [Line Items] |  |
Less than 12 months, Fair Value | 597 | 843
Less than 12 months, Gross Unrealized Losses | (2) | (3)
12 months or more, Fair Value | 42 | 52
12 months or more, Gross Unrealized Losses | (1) | (1)
Total, Fair Value | 639 | 895
Total, Gross Unrealized Losses | (3) | (4)
Debt Securities | Residential Mortgage-Backed Securities |  |
Investment [Line Items] |  |
Less than 12 months, Fair Value | 2 | 2
Less than 12 months, Gross Unrealized Losses | 0 | 0
12 months or more, Fair Value | 3 | 4
12 months or more, Gross Unrealized Losses | 0 | 0
Total, Fair Value | 5 | 6
Total, Gross Unrealized Losses | 0 | 0
Debt Securities | Commercial Mortgage-Backed Securities |  |
Investment [Line Items] |  |
Less than 12 months, Fair Value | 276 | 626
Less than 12 months, Gross Unrealized Losses | (6) | (13)
12 months or more, Fair Value | 249 | 265
12 months or more, Gross Unrealized Losses | (27) | (28)
Total, Fair Value | 525 | 891
Total, Gross Unrealized Losses | (33) | (41)
Debt Securities | Asset-backed securities |  |
Investment [Line Items] |  |
Less than 12 months, Fair Value | 146 | 258
Less than 12 months, Gross Unrealized Losses | 0 | (2)
12 months or more, Fair Value | 0 | 0
12 months or more, Gross Unrealized Losses | 0 | 0
Total, Fair Value | 146 | 258
Total, Gross Unrealized Losses | 0 | (2)
Debt Securities | Corporate debt securities |  |
Investment [Line Items] |  |
Less than 12 months, Fair Value | 374 | 918
Less than 12 months, Gross Unrealized Losses | (16) | (45)
12 months or more, Fair Value | 97 | 63
12 months or more, Gross Unrealized Losses | (11) | (10)
Total, Fair Value | 471 | 981
Total, Gross Unrealized Losses | (27) | (55)
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | U.S. Treasury and agency obligations |  |
Investment [Line Items] |  |
Less than 12 months, Fair Value | 76 | 195
Less than 12 months, Gross Unrealized Losses | 0 | (1)
12 months or more, Fair Value | 4 | 14
12 months or more, Gross Unrealized Losses | 0 | 0
Total, Fair Value | 80 | 209
Total, Gross Unrealized Losses | 0 | (1)
Debt Securities | U.S. Treasury And Other U.S. Government Corporations And Agencies | Mortgage-backed securities |  |
Investment [Line Items] |  |
Less than 12 months, Fair Value | 177 | 1,484
Less than 12 months, Gross Unrealized Losses | 0 | (20)
12 months or more, Fair Value | 126 | 86
12 months or more, Gross Unrealized Losses | (1) | (3)
Total, Fair Value | 303 | 1,570
Total, Gross Unrealized Losses | $ (1) | $ (23)

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