Variable Interest Entities - Summary of Assets and Liabilities Related to Consolidated Variable Interest Entities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Variable interest entities [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	$ 372	$ 380
Consolidated Liabilities	178	167
Other assets [Member] | Low income housing limited liability partnership [Member]
Variable Interest Entity [Line Items]
Consolidated Assets	372	380
Long-term debt [Member] | Low income housing limited liability partnership [Member]
Variable Interest Entity [Line Items]
Consolidated Liabilities	92	92
Interest, taxes and other liabilities [Member] | Low income housing limited liability partnership [Member]
Variable Interest Entity [Line Items]
Consolidated Liabilities	$ 86	$ 75