Plan Assets Measured at Fair Value on a Recurring Basis (Detail) - USD ($) $ in Millions		Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	$ 166	$ 195
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1],[2]	52	56
Mutual And Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	101	120
Mutual And Exchange Traded Funds | Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	15	7
Mutual And Exchange Traded Funds | International Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	35	38
Mutual And Exchange Traded Funds | Domestic Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	31	31
Mutual And Exchange Traded Funds | Debt Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	3	22
Mutual And Exchange Traded Funds | Alternative strategies
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	11	22
Mutual And Exchange Traded Funds | Commodity Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	6
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	13	19
Debt Securities | U.S. Treasury and Federal Agencies Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	4	3
Debt Securities | Agency Mortgage-Backed Securities | Residential mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	3	4
Debt Securities | Agency Mortgage-Backed Securities | Commercial mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	2	7
Debt Securities | Non-Agency Mortgage-Backed Securities | Commercial mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	1	2
Debt Securities | Asset-Backed Securities and Other Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1],[3]	3	3
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	75	66
Fair Value, Inputs, Level 1 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1],[2]	52	56
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	21	7
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds | Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	15	7
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds | Commodity Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	6
Fair Value, Inputs, Level 1 | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	2	3
Fair Value, Inputs, Level 1 | Debt Securities | U.S. Treasury and Federal Agencies Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	2	3
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	91	129
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	80	113
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | International Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	35	38
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | Domestic Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	31	31
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | Debt Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	3	22
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | Alternative strategies
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	11	22
Fair Value, Inputs, Level 2 | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	11	16
Fair Value, Inputs, Level 2 | Debt Securities | U.S. Treasury and Federal Agencies Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	2
Fair Value, Inputs, Level 2 | Debt Securities | Agency Mortgage-Backed Securities | Residential mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	3	4
Fair Value, Inputs, Level 2 | Debt Securities | Agency Mortgage-Backed Securities | Commercial mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	2	7
Fair Value, Inputs, Level 2 | Debt Securities | Non-Agency Mortgage-Backed Securities | Commercial mortgage backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1]	1	2
Fair Value, Inputs, Level 2 | Debt Securities | Asset-Backed Securities and Other Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	[1],[3]	$ 3	$ 3

[1] For further information on fair value hierarchy levels, refer to Note 1.

[2] Includes holdings in Bancorp common stock.

[3] Includes corporate bonds.