Plan Assets Measured at Fair Value on a Recurring Basis (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Defined Benefit Plan Disclosure [Line Items]
Securities	$ 195	[1]	$ 200	[1]
Equity securities
Defined Benefit Plan Disclosure [Line Items]
Securities	56	[1]	48	[1]
Equity securities | Value Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	10	[1]	8	[1]
Equity securities | Blended Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	46	[1],[2]	40	[1],[2]
Mutual And Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	120	[1]	134	[1]
Mutual And Exchange Traded Funds | Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	7	[1]	7	[1]
Mutual And Exchange Traded Funds | International Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	38	[1]	43	[1]
Mutual And Exchange Traded Funds | Domestic Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	31	[1]	41	[1]
Mutual And Exchange Traded Funds | Debt Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	22	[1]	20	[1]
Mutual And Exchange Traded Funds | Alternative Strategies Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	22	[1]	17	[1]
Mutual And Exchange Traded Funds | Commodities Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities			6	[1]
Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	19	[1]	18	[1]
Debt Securities | US Treasury Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	3	[1]	3	[1]
Debt Securities | Agency mortgage-backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	11	[1]	13	[1]
Debt Securities | Non Agency Residential Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	2	[1]	2	[1]
Debt Securities | Corporate Bond Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	3	[3]
Fair Value, Inputs, Level 1
Defined Benefit Plan Disclosure [Line Items]
Securities	66	[1]	64	[1]
Fair Value, Inputs, Level 1 | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Securities	56	[1]	48	[1]
Fair Value, Inputs, Level 1 | Equity securities | Value Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	10	[1]	8	[1]
Fair Value, Inputs, Level 1 | Equity securities | Blended Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	46	[1],[2]	40	[1],[2]
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	7	[1]	13	[1]
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds | Money Market Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	7	[1]	7	[1]
Fair Value, Inputs, Level 1 | Mutual And Exchange Traded Funds | Commodities Investment [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities			6	[1]
Fair Value, Inputs, Level 1 | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	3	[1]	3	[1]
Fair Value, Inputs, Level 1 | Debt Securities | US Treasury Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	3	[1]	3	[1]
Fair Value, Inputs, Level 2
Defined Benefit Plan Disclosure [Line Items]
Securities	129	[1]	136	[1]
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds
Defined Benefit Plan Disclosure [Line Items]
Securities	113	[1]	121	[1]
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | International Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	38	[1]	43	[1]
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | Domestic Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	31	[1]	41	[1]
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | Debt Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	22	[1]	20	[1]
Fair Value, Inputs, Level 2 | Mutual And Exchange Traded Funds | Alternative Strategies Fund [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	22	[1]	17	[1]
Fair Value, Inputs, Level 2 | Debt Securities
Defined Benefit Plan Disclosure [Line Items]
Securities	16	[1]	15	[1]
Fair Value, Inputs, Level 2 | Debt Securities | Agency mortgage-backed securities
Defined Benefit Plan Disclosure [Line Items]
Securities	11	[1]	13	[1]
Fair Value, Inputs, Level 2 | Debt Securities | Non Agency Residential Mortgage Backed Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	2	[1]	2	[1]
Fair Value, Inputs, Level 2 | Debt Securities | Corporate Bond Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Securities	$ 3	[3]

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

[2] Includes holdings in Bancorp common stock.

[3] Includes private label asset-backed securities.